2
You will need to edit the main menu template to do this, then adjust your style.css for the additional class -
<!-- Main Menu Template -->
<table cellspacing="0">
<tr>
<td id="mainmenu">
<a class="menuTop[color=ff0000]<{if $xoops_dirname=='system'}> actlink<{/if}>[/color]" href="<{$xoops_url}>/"><{$block.lang_home}></a>
<!-- start module menu loop -->
<{foreach item=module from=$block.modules}>
<a class="menuMain[color=ff0000]<{if $xoops_dirname==$module.directory}> actlink<{/if}>[/color]" href="<{$xoops_url}>/modules/<{$module.directory}>/"><{$module.name}></a>
<{foreach item=sublink from=$module.sublinks}>
<a class="menuSub" href="<{$sublink.url}>"><{$sublink.name}></a>
<{/foreach}>
<{/foreach}>
<!-- end module menu loop -->
</td>
</tr>
</table>
and in style.css -
.actlink {border-right: solid #0a1b51; background-color: #dde;}
In your case, the stylesheet would specify an image for the .actlink class
