Board Thread:General Discussion/@comment-11430210-20130717015040/@comment-4018164-20130723153528

Go to Special:ThemeDesigner in your wiki. In the "Customize" tab you can upload a background image that is smaller than 150KB.

In case you want to use a bigger file, add to your MediaWiki:Wikia.css the following code: And replace:
 * COLOR with a color name or a color value for your background.
 * FILEURL the URL of the image that you want to use as a background.