Paste this in your header template, and thats it. BTW what version vbulletin are you running?
PHP Code:
<!-- Header-->
<div align="center">
<table width="1200" border="0" cellpadding="0" cellspacing="0" class="border">
<td style="background-image:url($stylevar[imgdir_misc]/headerx.png); background-repeat:repeat-x;"><table width="1" border="0" align="center" cellpadding="0" cellspacing="0">
<tr>
<td valign="top"><a href="index.php"><img src="$stylevar[imgdir_misc]/header.png" alt="" width="1200" height="300" border="0" /></a></td>
</tr>
</table>
<!-- Header End-->
<!-- content table -->
$spacer_open
$_phpinclude_output
$ad_location[ad_header_end]