I actually just played around with this yesterday on my forum. What you want to do is the following...
You must apply these changes to
forumhome_forumbit_level2_post. forumhome_subforumbit_post is when you set the forum to display the subforums of a forum (like the small text under "Premium Modifications" on vb.org).
Code:
<tr align="center">
<td class="alt2"><img src="$stylevar[imgdir_statusicon]/forum_$forum[statusicon].gif" alt="" border="0" /></td>
<td class="alt1Active" align="$stylevar[left]" id="f$forum[forumid]">
<div>
<div style="float:left"><img src="your image"/></div>
<a href="forumdisplay.php?$session[sessionurl]f=$forum[forumid]"><strong>$forum[title]</strong></a>
<if condition="$show['browsers']"><span class="smallfont">(<phrase 1="$forum[browsers]">$vbphrase[x_viewing]</phrase>)</span></if>
</div>
<if condition="$show['forumdescription']"><div class="smallfont">$forum[description]</div></if>
<if condition="$show['forumsubscription']"><div class="smallfont"><strong><a href="subscription.php?$session[sessionurl]do=removesubscription&f=$forum[forumid]" rel="nofollow">$vbphrase[unsubscribe_from_this_forum]</a></strong></div></if>
<if condition="$show['subforums']"><div class="smallfont" style="margin-top:$stylevar[cellpadding]px"><strong>$vbphrase[subforums]</strong>: $forum[subforums]</div></if>
</td>
<td class="alt2" nowrap="nowrap">$forum[lastpostinfo]</td>
<td class="alt1">$forum[threadcount]</td>
<td class="alt2">$forum[replycount]</td>
<if condition="$vboptions['showmoderatorcolumn']">
<td class="alt1"><div class="smallfont">$forum[moderators] </div></td>
</if>
</tr>
$childforumbits
Basically add the green code into the proper template.
You will need to include conditionals on $forum[forumid] if you want to assign forum-specific images.