Use the Forum ID variable {$forum['fid']} to differentiate your banners.
So if your banner is an image for example, then rename the files with an additional number corresponding to your forum ID.
E.g.: banner1.png; banner2.png (where your Forum ID is 1 and 2).
Then add your img-Tag to the end of forumbit template as follows:
<img src="banner{$forum['fid']}.png" ...>
That's it
[ExiTuS]
Edit:
Another option is to add an iframe to the template that displays banners (maybe rotating banners by script).
[ExiTuS]
I use plugin myadvertisment my php now is :
<table border="0" cellspacing="{$theme['borderwidth']}" cellpadding="{$theme['tablespace']}" class="tborder">
<thead>
<tr>
<td class="thead{$expthead}" colspan="5">
<div class="expcolimage"><img src="{$theme['imgdir']}/{$expcolimage}" id="cat_{$forum['fid']}_img" class="expander" alt="{$expaltext}" title="{$expaltext}" /></div>
<div><strong><a href="{$forum_url}">{$forum['name']}</a></strong><br /><div class="smalltext">{$forum['description']}</div></div>
</td>
</tr>
</thead>
<tbody style="{$expdisplay}" id="cat_{$forum['fid']}_e">
<tr>
<td class="tcat" colspan="2"><span class="smalltext"><strong>{$lang->forumbit_forum}</strong></span></td>
<td class="tcat" width="85" align="center" style="white-space: nowrap"><span class="smalltext"><strong>{$lang->forumbit_threads}</strong></span></td>
<td class="tcat" width="85" align="center" style="white-space: nowrap"><span class="smalltext"><strong>{$lang->forumbit_posts}</strong></span></td>
<td class="tcat" width="200" align="center"><span class="smalltext"><strong>{$lang->forumbit_lastpost}</strong></span></td>
</tr>
{$sub_forums}
</tbody>
</table>
<br />
{myadvertisements[zone_10]}
I use to do this but didn't work
<if $foruminfo['fid'] ==1 then>
<img src="{myadvertisements[zone_10]}">
</if>
This is work fine
<if $forum['fid'] == 1 then>
<center><div>{myadvertisements[zone_13]}</div></center>
</if>