Board Thread:Support Requests - Getting Technical/@comment-11842624-20150111215303/@comment-1757994-20171103224152

If you're trying to set a background image with an inline style, MediaWiki strips that out as a potential security violation.

If you're trying to set a background image from a local source in a CSS file, your browser is probably blocking it. I cannot speak authoritatively on all versions of all browsers everywhere, but most will puke all over mixing local and remote sources on the same page.