If you had that background show up out of nowhere I wouldn't know what to tell you, that is pretty odd, even with a virus.
Did you have that pic of that smoking forum already somewhere on your computer?
What kind of anti-virus program are you running and do you run any adware/spyware programs?
Any chance you are running peerGuardian or No Scripts? (no scripts is firefox only, not internet explorer)
Anyone else in the household that would have swapped your background without telling you?
Even if there was a virus and the forum had any cookies/hits in your temp or history files I don't know of anything or even any good reason why a virus would swap your background and nothing else.
I would suggest running a virus scan, scan for adware, see what the background file for your desktop is called, locate that file and see where it is stored.