uplaod file to images/misc in you forum directory then goto admincp, now goto styles & templates and click the drop down box next to the style you want to edit, then click edit templates now scroll thru the list untill you come to navigation and breadcrumb templates, double click then double click navbar
find
<script type="text/javascript">
<!--
function log_out()
{
ht = document.getElementsByTagName("html");
ht[0].style.filter = "progid:DXImageTransform.Microsoft.BasicImage(grays cale=1)";
if (confirm('$vbphrase[sure_you_want_to_log_out]'))
{
return true;
}
else
{
ht[0].style.filter = "";
return false;
}
}
//-->
</script>
then after add
<!-- Banner -->
<br />
<center>
<table align="center"><a href="LINK LOCATION" target="_blank"><img src="$stylevar[imgdir_misc]/IMAGENAME.gif" border="0" alt="" /></a>
</center>
</table>
<!-- /Banner -->
change the LINK LOCATION and IMAGENAME to your desire
then click save and your done.
Hope that helps ;)