In the settings of modloader, there is a textbox setting for "Background Image Url". This lets you choose a background image for the site as long as the image is hosted somewhere on the web. So you won't be able to change the background if you only have the image as a file on your computer (unless you upload it to an image hosting website, or convert it to a data URI first).
- Find an image you like. These are some good stock/wallpaper sites to try:
- Right click on the image and do "copy image address"
- Then paste into the modloader setting for "Background Image Url"
- Hit apply
