Board Thread:Support Requests - Designing Your Wikia/@comment-26490192-20150618191054/@comment-12613595-20150618194220

I think adding a background is exclusive to the Theme Designer feature. I don´t think it can be done with coding.... I´ve always used the Theme Designer and always answered to questions with it. Anyway, if it can be done, that code would probably not work.

My advice is still use the Theme Designer. It´s the simplest way. To reduce the KB size of your image, which as you said is over 300 KB, you can just compress it or change it´s format. Sure it looses a bit of it´s quality in the process but it doesn´t change that much. Those are both really simple to perform.