After making sure your images are 150X30 check menu.tpl
Try this
Code:
<TABLE border=0 cellPadding=5 cellSpacing=0 width="100%">
<TR>
{if $link_href}
<TD class=VertMenuTitle background="{$ImagesDir}/your_image.gif" height=30 valign=center width=150>
<font class=VertMenuTitle>{$menu_title}</font></TD>
{else}
<TD class=VertMenuTitle background="{$ImagesDir}/your_image.gif" height=30 valign=center width=150><font class=VertMenuBoxNewsTitle>{$menu_title}</font></TD>
{/if}
</TR>
<TR>
<TD colspan="2" class=VertMenuBox>{$menu_content}
</TD></TR>
</TABLE>
change your_image.gif to the name of your image file