In your navbar find:
HTML Code:
<div class="navbar" style="font-size:10pt"><a href="$vboptions[forumhome].php$session[sessionurl_q]" accesskey="1"><img class="inlineimg" src="$stylevar[imgdir_misc]/navbits_start.gif" alt="" border="0" /></a> <strong>$vboptions[bbtitle]</strong></div>
</if>
</td>
After that add:
HTML Code:
<td class="alt2">
<img src="your banners image url" alt="" border="0" />
</td>