[tonjohn]

Inspecting the example you gave with Firebug, you see the following (one forum in the list used as an example...

<li class="forumbit_post old L2" id="forum61">
	<div class="forumrow table">
		<div class="foruminfo td">
			<img alt="" id="forum_statusicon_61" class="forumicon" src="images/statusicon/forum_old-48.png">
			<div class="forumdata">
				<div class="datacontainer">
					<div class="titleline">
						<h2 class="forumtitle"><a href="http://www.luxury4play.com/ferrari/">Ferrari</a></h2>
						<span class="viewing">(23 Viewing)</span>
					</div>
					

					

					

					
				</div>
			</div>
		</div>
        
		<h4 class="nocss_label">Forum Actions:</h4>
        	
		<ul class="forumactionlinks td">
			<li class="forumactionlink rsslink"><a title="View this forum's RSS feed" href="http://www.luxury4play.com/external.php?type=RSS2&amp;forumids=61" rel="nofollow">View this forum's RSS feed</a></li>
			
		</ul>
		
        <h4 class="nocss_label">Forum Statistics:</h4>
		<ul class="forumstats td">
			<li>Threads: 2,941</li>
			<li>Posts: 61,038</li>
		</ul>
		<div class="forumlastpost td">
			<h4 class="lastpostlabel">Last Post:</h4>
			<div>
				
	<p class="lastposttitle">
	<img border="0" alt="" src="images/icons/icon1.png">
	
	<a class="threadtitle" href="http://www.luxury4play.com/ferrari/93512-ferraris-sema.html">The Ferraris of SEMA</a>
	<a href="http://www.luxury4play.com/ferrari/93512-ferraris-sema-post1567167.html#post1567167"><img alt="Go to last post" src="images/buttons/lastpost-right.png" title="Go to last post"></a>
	</p>
	<div class="lastpostby">
	
		by <div class="popupmenu memberaction" id="yui-gen12">
	<a title="Phaster is online now" href="http://www.luxury4play.com/members/phaster.html" class="username online "><strong>Phaster</strong></a>
	
</div>

	
	</div>
	<p class="lastpostdate">Yesterday, <span class="time">11:23 PM</span></p>

			</div>
		</div>
        
	</div>
	
</li>

The corresponding CSS to draw the car logo is:

#forum61 .foruminfo {
    background-image: url("/images/l4p/forumheads/Ferrari.jpg") !important;
    background-position: right center;
    background-repeat: no-repeat;
}

The CSS doesn't need to compute the forumids. Rather, you simply need to create an id in your CSS file to correspond to every forum you wish to show a unique image for.

You then edit the correct templates (forumhome_forumit_), adding id=forum$forum[forumid] (not exactly sure) to the <li> tag.