you can do an if condition i think.. if forumid is equal to the forum you want the banner displayed in, then the banner is shown.. if not, nothing is displayed..
this is kinda modified from what i have on my forum.. but maybe this will work.. someone may know better than i, though.. and this is in the header template beside the forum logo image..
HTML Code:
<if condition=" forumid == X ">
<!-- banner insert -->
<td align="right">
<img src="http://pathtobanner.com/imageofbanner.jpg">
</td>
<!-- /banner insert -->
</if>