I have done something similar. Using if -> then statements, you can display a banner in the forum using forum_ID. You can get the forum ID number from FORUMS > LIST ALL > DETAILS (for the specific forum)
I think this should work if you insert it into TEMPLATES > USERS TEMPLATES > forum_view.html just under :
<%endloop%>
<%endif%>
<%/body_font%>
</td>
<td>
<%include include_logo.html%>
</td>
</tr>
at the top of that template.
<%if forum_id >= 1><tr>
<td colspan="3">
<center>
<img src="
http://www.yoursite.com/ads/ad1.jpg">
</center>
</td>
</tr>
<%endif%>
best of luck to you. I went a step further to make mine work and it was probably more difficult than it was worth. I went to TOOLS > TABLE EDITOR > FORUMS and added a column for some value and gave the one forum that I wanted the special graphic in a 1 value - so my if statement did not have an equal in it - it reads:
<%if team%>
essentially meaning if that forum has a value of 1 in the column named team, it would do the sub routine to add that graphic - the other forums have a 0 in that field. But if you were to have people sponsor different forums, the easiest way would be to utilize the code above so that you could do multiple forums with different sponsors. for instance:
<%if forum_id >= 1><tr> <td colspan="3">
<center>
<img src="
http://www.yoursite.com/ads/ad1.jpg">
</center>
</td>
</tr>
<%endif%>
<%if forum_id >= 2><tr> <td colspan="3">
<center>
<img src="
http://www.yoursite.com/ads/ad2.jpg">
</center>
</td>
</tr>
<%endif%>
<%if forum_id >= 3><tr> <td colspan="3">
<center>
<img src="
http://www.yoursite.com/ads/ad3.jpg"> </center>
</td>
</tr>
<%endif%>