That's a hack I've never seen before, albeit a simple one.
They simply added background="$NonCGIURL/Forum$x/background.jpg"
(or appropriate HTML coding) to the ForumGutsHTML subroutine in Ultimate.cgi, or so it would appear.
As this is a code hacking issue, any followups do belong in Code Hacking.
Moving there.. <img src="https://www.ubbcentral.com/boards/images/graemlins/smile.gif" alt="" />
Graeme