I have something similar but the middle strip is the repeating part while my left and right images stay put. Here is my header template, most of it anyway.
Code:
<!-- logo -->
<a name="top"></a>
<center>
<table border="0" width="$stylevar[outertablewidth]" cellpadding="0" cellspacing="0" align="center" style="background-image:url($stylevar[imgdir_misc]/bkg_header_blu.png)">
<tr>
<td align="left" valign="top"><a href="$vboptions[forumhome].php$session[sessionurl_q]"><img src="$stylevar[imgdir_misc]/left_header_blu.png" border="0" alt=""/></a></td>
<td align="right" valign="bottom"><img src="$stylevar[imgdir_misc]/right_header_blu.png" alt="" /></td>
</tr>
</table>
</center>
My bkg_header_blu.png is the one that's only a few pixels wide.
This works for me perfectly. Hope it helps? My forum is fluid and I've used this in all my styles. It is here:
www.dooberville.com/forums