Under the skin1 directory you will see menu.tpl go inside there and add you image tag to it and you will now see the background image. Mine looks like this now.
Code:
<TD class=VertMenuBox background="{$ImagesDir}/tab2.gif" height=26>{$link_begin}[img]{$ImagesDir}/{$dingbats}[/img]{$link_end}</TD>
Hope this is what your looking for.