Not sure if you can get different backgrounds for each forum title with out redoing alot of templates. If you just want to add a background for all the forum titles just edit the template " forumhome_forumbit_level1_nopost " and make it look somthing like this:
<tr id="cat">
<td background="images/layout/forum_bg.jpg" bgcolor="#336D95" background="images/catagory_backing.gif" bgcolor="#336D95" background="images/catagory_backing.gif" colspan="6"><a href="forumdisplay.php?s=$session[sessionhash]&forumid=$forum[forumid]"><normalfont color="#000000"><b>$forum[title]</b></normalfont></a>
<br><smallfont color="#000000">$forum[description]</smallfont></td>
</tr>
Just change " background="images/layout/forum_bg.jpg " to the image you want to use.
You may also want to edit the template "forumhome_forumbit_level2_post"
I know this isnt the answer you were looking for but it may help you in getting what you want