thats the easy way :
in your navbar template find this :
<else />
<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 add :
<td class="alt1" width="100%">
<a href="yourbannerurl"><img src="yourbannerimageurl" border="0" /></a>
</td>
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if you don't want any border between 2 td spaces, you must do this :
find this :
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center">
<tr>
<td class="alt1" width="100%"><if condition="is_array($navbits)">
<table cellpadding="0" cellspacing="0" border="0">
<tr valign="bottom">
<td><a href="#" onclick="history.back(1); return false;"><img src="$stylevar[imgdir_misc]/navbits_start.gif" alt="$vbphrase[go_back]" border="0" /></a></td>
<td> </td>
<td width="100%"><span class="navbar"><a href="$vboptions[forumhome].php$session[sessionurl_q]" accesskey="1">$vboptions[bbtitle]</a></span> $navbits[breadcrumb]</td>
</tr>
<tr>
<td class="navbar" style="font-size:10pt; padding-top:1px" colspan="3"><if condition="$_SERVER['REQUEST_METHOD'] == 'POST'"><img class="inlineimg" src="$stylevar[imgdir_misc]/navbits_finallink.gif" alt="" border="0" /><else /><a href="$scriptpath"><img class="inlineimg" src="$stylevar[imgdir_misc]/navbits_finallink.gif" alt="$vbphrase[reload_this_page]" border="0" /></a></if> <strong>$navbits[lastelement]</strong></td>
</tr>
</table>
<else />
<div class="navbar" style="font-size:10pt"><a href="$vboptions[forumhome].php$session[sessionurl_q]" accesskey="1"><img class="inlineimg" src="$stylevar[imgdir_misc]/navbits_start.gif" alt="" border="0" /></a> <strong>$vboptions[bbtitle]</strong></div>
</if>
replace with :
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center">
<tr>
<td class="alt1" width="100%" valign="top" align="left">
<table width="100%" border="0" cellspacing="0" cellpadding="0">
<tr>
<td><if condition="is_array($navbits)">
<table cellpadding="0" cellspacing="0" border="0">
<tr valign="bottom">
<td><a href="#" onclick="history.back(1); return false;"><img src="$stylevar[imgdir_misc]/navbits_start.gif" alt="$vbphrase[go_back]" border="0" /></a></td>
<td> </td>
<td width="100%"><span class="navbar"><a href="$vboptions[forumhome].php$session[sessionurl_q]" accesskey="1">$vboptions[bbtitle]</a></span> $navbits[breadcrumb]</td>
</tr>
<tr>
<td class="navbar" style="font-size:10pt; padding-top:1px" colspan="3"><if condition="$_SERVER['REQUEST_METHOD'] == 'POST'"><img class="inlineimg" src="$stylevar[imgdir_misc]/navbits_finallink.gif" alt="" border="0" /><else /><a href="$scriptpath"><img class="inlineimg" src="$stylevar[imgdir_misc]/navbits_finallink.gif" alt="$vbphrase[reload_this_page]" border="0" /></a></if> <strong>$navbits[lastelement]</strong></td>
</tr>
</table>
<else />
<div class="navbar" style="font-size:10pt"><a href="$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>
<td valign="top" align="right"><a href="yourbannerurl"><img src="yourbannerimageurl" border="0" /></a> </td>
</tr>
</table>
* you have to edit <a href="yourbannerurl"><img src="yourbannerimageurl" border="0" /></a> for your settings..