Board Thread:Support Requests - Designing Your Wiki/@comment-31714129-20191014205709/@comment-168424-20191014214755

As far as I know, what you want can't be done without javascript. Background images can't be applied dynamically with just CSS and also can't be done in inline CSS. You can try asking at on the  if they have any ideas.

All that said, I have no idea how you are currently doing it, so I can't really say for sure.