Asynchronous thumbnail creation and optimization for Kirby CMS.
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GitHub release License Kirby Version

ImageKit provides an asynchronous thumbnail API and advanced image optimization for Kirby CMS.

NOTE: This is not be a free plugin. In order to use it on a production server, you need to buy a license. For details on ImageKit’s license model, scroll down to the License section of this document.

ImageKit’s Dashboard Widget

Table of Contents

1 Key Features

  • Image-resizing on demand: Kirby’s built-in thumbnail engine resizes images on-the-fly while executing the code in your template files. On image-heavy pages, the first page-load can take very long or even exceed the maximum execution time of PHP. ImageKit resizes images only on-demand as soon as they are requested by the client.
  • Optimization: ImageKit can utilize several command-line utilities to apply superior compression to your images. Both lossless and lossy optimizers are available. Your images look as shiny as before, but your pages will load much faster.*
  • Security: A lot of thumbnail libraries for PHP still offer the generation of resized images through URL parameters (e.g. thumbnail.php?file=ambrosia.jpg&width=500), which is a potential vector for DoS attacks. ImageKit only generates the thumbnails whose are defined in your page templates.
  • Widget: Pre-Generate your thumbnails right from the panel with a single click.
  • Discovery Feature: The widget scans you whole site for new thumbnails, before creating them.
  • Error-Handling: ImageKit let’s you know, when errors occur during thumbnail creation (experimental).
  • Self-Hosted: Unlike many other image-resizing-services, ImageKit just sits in Kirby’s plugin directory, so you have everything under control without depending on external providers. No monthly fees. No visitor data is exposed to external companies. tl;dr: No bullshit!

*) To use optimization features, your need to have the corresponding command-line utilities installed on your server or have sufficient permissions to install them. The effectiveness of compression also depends heavily on your source images.

2 Download and Installation

2.1 Requirements

  • PHP 5.4.0+ (With libxml if you’re using discovery mode. This extension is usually installed on most hosting providers.)
  • Kirby 2.3.0+
  • GD Library for PHP or ImageMagick command-line tools to resize images.
  • Tested on Apache 2 with mod_rewrite (but it should also work with other servers like nginx)
  • Permission to install and execute several command-line utilities on your server, if your want to use the optimization feature.

2.2 Kirby CLI

If you’re using the Kirby CLI, you need to cd to the root directory of your Kirby installation and run the following command:

kirby plugin:install fabianmichael/kirby-imagekit

This will download and copy ImageKit into site/plugins/imagekit.

2.3 Git Submodule

To install this plugin as a git submodule, execute the following command from the root of your kirby project:

$ git submodule add site/plugins/imagekit

2.4 Copy and Paste

  1. Download the contents of this repository as ZIP-file.
  2. Rename the extracted folder to imagekit and copy it into the site/plugins/ directory in your Kirby project.

3 Usage

Just use it like the built-in thumbnail API of Kirby. You can learn more about Kirby’s image processing capabilities in the Kirby Docs.

Due to the fact that thumbs created by ImageKit remain virtual until the the actual thumb file has been requested by a visitor of your website, some API methods will trigger instant creation of a thumbnail. You should avoid to call methods like size(), base64() or modified() on your thumb, whenever possible, because they only work after the actual thumbnail has been created. However, you can safely use methods like width(), height() and ratio() because dimensions are calculated prior to thumbnail creation.

If you don’t want to let the first visitors of your site need to wait for images to appear, all thumbnails on your site can be generated from ImageKit’s dashboard widget in advance.

4 How it works

Rather than doing the expensive task of image conversion on page load (default behavior of Kirby’s built-in thumbs API), thumbnails are stored as a »job« instead as the API is called by your template code. So they will only be generated, when a particular image size is requested by the browser. ImageKit also comes with a widget, so you can trigger creation of all thumbnails right from the panel.

4.1 Discovery mode

If the (automatic indexing) option is active, the widget will not only scan your thumbs folder for pending thumbnails, but will also make a HTTP request to every single page of your Kirby installation to execute every page‘s template code once. This feature also works with pagination and/or prev- and next links. Just make sure, that the pagination links have rel attributes of either 'next' or 'prev'. This way, ImageKit can even scan through paginated pages.

<link href="<?= $pagination->prevPageURL() ?>" rel="prev">
<link href="<?= $pagination->nextPageURL() ?>" rel="next">
<a href="<?= $pagination->prevPageURL() ?>" rel="prev">Previous page</a>
<a href="<?= $pagination->nextPageURL() ?>" rel="next">Next page</a>

This currently works by using PHP’s DOM interface (DOMDocument), so if your HTML contains a lot of errors, this might fail. If you are experiencing any trouble with this feature, please report a bug so I can make it work with your project.

5 Basic Configuration

Option Default value Description
imagekit.license '' Enter your license code here, once your site goes live.
See the License section of this document for more information.
imagekit.lazy true Set to false to temporary disable asynchronous thumbnail generation. This will restore the default behavior of Kirby.
imagekit.complain true If enabled, ImageKit will try to return a placeholder showing an error symbol whenever thumbnail creation fails. If you don’t like this behavior, you can turn this feature off and ImageKit will fail silently.
imagekit.widget true Enables the dashboard widget.
imagekit.widget.step 5 Sets how many pending thumbnails will be generated by the widget in one step. If thumbnail generation exceeds the max execution time on your server, you should set this to a lower value. If your server is blazingly fast, you can safely increase the value. true If enabled, the widget scans your whole site before creating thumbnails. If this feature is not compatible with your setup, disable it. It can also take very long on large site, every single page has to be rendered in order to get all pending thumbnails. In order to do this, the plugin will flush your site cache before running.

6 Image Optimization

6.1 Setup

As of version 1.1, ImageKit is also capable of optimizing thumbnails. There are different optimizers, providing both lossless and lossy optimization. It works similar to third-party services (e.g. TinyPNG,, but on your own server. If you want to use this feature, you have to install the corresponding command-line utilities first—if they’re not already installed on your server.

ℹ️ If your hosting provider doesn’t let you compile software on your webspace (most likely for shared hosting), you can get binaries for most operation systems from a WordPress plugin called EWWW-Image-Optimizer. Just download the plugin and upload it’s the pre-compiled utilities to your server (look into the folder binaries within the ZIP archive). There are many other places, where you can get pre-compiled versions of the image optimation tools, but please be careful and do not download any of these tools from some strange russian server. The only tool I couldn’t find as a pre-compiled binary for Linux / OS X hosts is mozjpeg.

Option Default value Description
imagekit.optimize false Set to true to enable optimization. In addition, you have to configure at least one of the optimizers listed below.
imagekit.driver null This option only needs to be set, if your Kirby installation uses a custom ImageMagick driver, that has a different name than 'im'. If this is the case, set this value to 'im' to tell ImageKit, that you’re using ImageMagick for thumbnail processing, as there is no other way to detect this reliably. In most cases, you can safely ignore this setting.
This settings does not change the thumbs driver to ImageMagick! If you want to use ImageMagick as your image processing backend, please refer to the corresponding pages in the Kirby documentation or have a look into the troubleshooting section of this document.

By default, all optimizers will be loaded and ImageKit checks if you have set the path to a valid binary (e.g. imagekit.mozjpeg.bin). If the binary is set and executable, ImageKit will then activate the optimizer automatically for supported image types. All optimizers come with a sane default configuration, but you can tweak them according to your needs.

6.2 Overriding Global Settings

If you need different optimization configuration settings for different images, you can override any of the settings (except for the path to an optimizers’s binary) you defined in config.php by passing them to the thumb() method. The parameter imagekit.optimize can also take an array of optimizers. Note, that optimizers will only become active, if the input image is in a format supported by them (e.g. If you provide a JPEG to the example below, pngquant will be skipped, because it can only handle PNG images.)

  'width'                    => 600,
  'imagekit.optimize'        => ['mozjpeg', 'pngquant'],
  'imagekit.mozjpeg.quality' => 60,

Overriding global settings might become useful, if you want to apply lossless optimization for some images and lossy optimization for others. A typical use-case would be a photo gallery with lots of small preview images on an index page, where you want to squeeze the last byte out of your thumbnails using mozjpeg. For the enlarged view of a photo, image quality might be more important than filesize, so you might prefer jpegtran over mozjpeg for lossless optimization.

6.3 Available Optimizers

6.3.1 mozjpeg

Mozjpeg is an improved JPEG encoder that produces much smaller images at a similar perceived quality as those created by GD Library, ImageMagick, or Photoshop. I really recommend to try out this optimizer, because it can significantly reduce the size of your thumbnails.

Download mozjpeg

Option Default value Possible Values Description
imagekit.mozjpeg.bin null Enter the path to mozjpeg’s encoder executable (cjpeg) to activate this optimizer.
(tested with mozjpeg 3.1)
imagekit.mozjpeg.quality 85 0-100 Sets the quality level of the generated image. Choose from a scale between 0-100, where 100 is the highest quality level. The scale is not identical to that of other JPEG encoders, so you should try different settings and compare the results if you want to get the optimal results for your project.
imagekit.mozjpeg.flags '' Use this parameter to pass additional options to the optimizer. Have a look at mozjpeg’s documentation for available flags.

ℹ️ I recommend that you don’t upscale images that have been compressed by mozjpeg, bacause it will add a lot of artifacts to thumbnails. Those are mostly invisible when the image is viewed at full size or downscaled. But they can give your images an unpleasant look, if they’re upscaled.

6.3.2 jpegtran

Jpegtran applies lossless compression to your thumbnails by optimizing the JPEG data and stripping out metadata like EXIF. If you use mozjpeg, there is no reason to also use jpegtran, as my tests did not show any benefit in thumbnail size, when both are used together.

Download jpegtran

Option Default value Possible Values Description
imagekit.jpegtran.bin null Enter the path to the optipng executable to activate this optimizer.
(tested with jpegtran 0.7.6)
imagekit.jpegtran.optimize true true, false Enables lossless optimization of image data.
imagekit.jpegtran.copy 'none' 'all', 'comments', 'none' Sets which metadata should be copied from source file.
imagekit.jpegtran.flags '' Use this parameter to pass additional options to the optimizer. Have a look at jpegtran’s documentation for available flags.

6.3.3 pngquant

Pngquant performs lossy optimization on PNG images by converting 24-bit images to indexed color (8-bit), while alpha-transparency is kept. The files can be displayed in all modern browsers and this kind of lossy optimization works great for most non-photographic images and screenshots. You may notice some color shifts on photographic images with a lot of different colors (you usually should not use PNG for displaying photos on the web anyway …).

Download pngquant

Option Default value Possible Values Description
imagekit.pngquant.bin null Enter the path to the pngquant executable to activate this optimizer.
(tested with optipng 2.7.2)
imagekit.pngquant.quality null null, 'min-max' (e.g. '0-100') Sets minimum and maximum quality of the resulting image. Has to be a string.
imagekit.pngquant.speed 3 1 = slow,
3 = default,
11 = fast & rough
Slower speed means a better quality, but optimization takes longer (for large images from a few megapixels and above, we’re talking about tens of seconds or even minutes, when using a speed setting of 1).
imagekit.pngquant.posterize false false,
Output lower-precision color if set to further reduce filesize.
imagekit.pngquant.colors false false, 2-256 Sets the number of colors for optimized images. Less colors mean smaller images, but also reduction of quality.
imagekit.pngquant.flags '' Use this parameter to pass additional options to the optimizer. Have a look at pngquant’s documentation for available flags.

6.3.4 optipng

Optipng performs lossless optimizations on PNG images by stripping meta data and optimizing the PNG data itself.

Download optipng

Option Default value Possible Values Description
imagekit.optipng.bin null Enter the path to the optipng executable to activate this optimizer.
(tested with optipng 0.7.6)
imagekit.optipng.level 2 0-7 Sets the optimization level. Note, that a high optimization level can make processing of large image files very slow, while having only little impact on filesize.
imagekit.optipng.strip 'all' 'all', false Strips all metadata from the PNG file.
imagekit.optipng.flags '' Use this parameter to pass additional options to the optimizer. Have a look at optipng’s documentation for available flags.

6.3.5 gifsicle

Gifsicle optimizes the data of GIF images. Especially for animations, using this optimizer can lead to a great improvement in file size, but can also take very long for large animations. Static GIF images will also benefit from using Gifsicle.

Download gifsicle

Option Default value Possible Values Description
imagekit.gifsicle.bin null Enter the path to the optipng executable to activate this optimizer.
(tested with optipng 1.88)
imagekit.gifsicle.level 3 false, 1-3 Sets the level of optimization, where 3 is the highest possible value.
imagekit.gifsicle.colors false false, 2-256 Sets the amount of colors in the resulting thumbnail. By default, color palettes are not reduced.
imagekit.gifsicle.flags '' Use this parameter to pass additional options to the optimizer. Have a look at gifsicle’s documentation for available flags.

7 Troubleshooting

How can I activate ImageMagick? As ImageKit acts as a proxy for Kirby’s built-in thumbnail engine, you have to activate it on your `config.php` file, just as you would do without ImageKit like below:
c::set('thumbs.bin', '/usr/local/bin/convert');

→ Kirby documentation for thumbs.driver and thumbs.bin

Please note, that Kirby uses the command-line version of ImageMagick, rather than its PHP extension. In order to use ImageMagick as your processing backend, the ImageMagick executable (convert) has to be installed on your server.*

Thumbnail creation always fails … This may happen because of several reasons. First, make sure that your thumbs folder is writable for Kirby. If you’re using the GD Library driver, make sure that PHP’s memory limit is set to a high-enough value. Increasing the memory limit allows GD to process larger source files. Or if you favor ImageMagick (I do), make sure that the path to the `convert` executable is correctly configured.
The Discovery Feature does not work with my site: Discovery works by creating a sitemap of your entire site and then sends an HTTP request to every of those URLs to trigger rendering of every single page. When doing so, ImageKit sees everything from a logged-in user’s perspective. It tries it’s best to find pagination on pages, but it cannot create thumbnails whose are – for example – only available on a search results page, where entries are only displayed when a certain keyword was entered into a form. Also make sure, that your Server’s PHP installation comes with `libxml`, which is used by PHP’s DOM interface.
Can I also optimize the images in my content folder? This is currently not possible, because it would need a whole UI for the admin panel and would also be very risky to apply some bulk processing on your source images without knowing the actual results of optimization. If you need optimized images in your content folder, I really recommend that you use tools like ImageOptim and ImageAlpha to optimize your images prior to uploading them. This saves space on your server and also speeds up your backups.
404 Errors with nginx ImageKit may have problems with certain nginx configurations, resulting 404 errors, when a thumbnail is requested for the first time. See this issue to learn, how you have to configure nginx to solve this issue.

8 License

ImageKit can be evaluated as long as you want on how many private servers you want. To deploy ImageKit on any public server, you need to buy a license. See for terms and conditions.

The plugin is also available as a bundle with ImageSet, a plugin for bringing responsive images with superpowers to you Kirby-driven site.

Buy ImageKit
Buy the ImageKit + ImageSet Bundle

However, even with a valid license code, it is discouraged to use it in any project, that promotes racism, sexism, homophobia, animal abuse or any other form of hate-speech.

9 Technical Support

Technical support is provided via Email and on GitHub. If you’re facing any problems with running or setting up ImageKit, please send your request to or create a new issue in this GitHub repository. No representations or guarantees are made regarding the response time in which support questions are answered.

10 Credits

ImageKit is developed and maintained by Fabian Michael, a graphic designer & web developer from Germany.