[Analogpoint]

Where the link is output in the templates, put in a conditional. Here's the pseudocode. Something along these lines. I didn't check what variables you would actually use. You'll have to take a look at these templates:

forumhome_forumbit_level1_nopost
forumhome_forumbit_level1_post
forumhome_forumbit_level2_nopost
forumhome_forumbit_level2_post
forumhome_subforumbit_nopost
forumhome_subforumbit_post

<a href="forumdisplay.php?f=$foruminfo[forumid]">
    <if condition="$foruminfo[forumid] == FORUMID">
           <!-- show banner -->
           <img src="path/to/image.jpg" alt="" /> 
   <else />
       <!-- show regular link text -->
       $foruminfo[title_clean]
    </if>
</a>