Board Thread:Support Requests - Designing Your Wiki/@comment-39286675-20200421013238/@comment-9605025-20200421043322

It depends on what you want that image to be. The main background (what you see on the sides) does in fact appear behind the article but may be hidden due to the transparency of the article's background. If you simply want to see the middle section of that image, then just use ThemeDesigner to increase the transparency of the article's background. If, on the other hand, you want it to be a different image, then you need to use site CSS as UpnCbs06 said.