I believe it would be in forumhome_forumbit_level2_post, the line would be:
<td bgcolor="{firstaltcolor}" align="left"><a href="forumdisplay.php?s=$session[sessionhash]&forumid=$forum[forumid]"><normalfont><b>$forum[title]</b></normalfont></a>
I changed bgcolor="{firstaltcolor}" to bgcolor="#ffffff" just to see where the change would be made, and it would have the same effect. Replace the bgcolor with backround="image" and I think it'll work.
Then again, Im a moron to this stuff, so always assume Im totally wrong, then point and laugh at me saying how totally wrong I am, and let C-Prompt or Sinecure come in here and tell you the correct way. :)