Board Thread:Support Requests - Designing Your Wikia/@comment-28122555-20140531191253/@comment-856287-20140601013746

You'll have to use "messy codes." There's no other way, but this change is pretty simple!

Go to your Wiki's Special:CSS and add the following: Put your background image's URL in there, save the page, and it will start to appear in the wiki header after caches update (a maximum of ten minutes).