There are many ways to export your images into tiles for Gigapixel, but the simplest way is to use PhotoShop.
To do this, first, open the image you want to use as your Gigapixel background image in PhotoShop.
Next, go to Image in the menu bar. Click on "Image Size" and change the size of your image. It is recommended that you have either a width or height of 10,000px. Make sure the image size is in pixels.
Save your image. Then, on the menu bar, click File. Choose Export, and then Zoomify.
PhotoShop will create three files: a folder named after your image, an HTML file and ZoomifyImageViewer-min.js.
Put these files onto your web server. When you link to the custom background in Gigapixel, make sure the URL links to the folder that Zoomify generated. The tiles will be found inside this folder