Board Thread:Support Requests - Getting Technical/@comment-35084100-20180501031352/@comment-35084100-20180501222537

Andrewds1021 wrote: You need to use the wiki's site-wide CSS. You can find information about the classes used by portable infoboxes at Help:Infoboxes/CSS.

There's no info on adding a background image to the infobox on the page you linked.