If you've got the default vBulletin header this should work:
HTML Code:
<!-- logo -->
<a name="top"></a>
<table border="0" width="$stylevar[outertablewidth]" cellpadding="0" cellspacing="0" align="center">
<tr>
<td align="$stylevar[left]"><a href="$vboptions[forumhome].php$session[sessionurl_q]"><img src="$stylevar[titleimage]" border="0" alt="$vboptions[bbtitle]" /></a></td>
<td align="center"> <!-- banner --><img src="path/to/banner.gif" border="0" align="middle"a /><!--/banner -->
</td>
</tr>
</table>
<!-- /logo -->
<!-- content table -->
$spacer_open
$_phpinclude_output
Hope this helps