This is a bit of a hack to get this going, but it works!

The easier version requires Google SEO. From your past threads it seems you already have it installed

.
Open Admin CP
Themes > your theme > templates > forumbit templates > forumbit_depth2_forum
Find:
<img src="{$theme['imgdir']}/{$lightbulb['folder']}.gif" alt="{$lightbulb['altonoff']}" title="{$lightbulb['altonoff']}" class="ajax_mark_read" id="mark_read_{$forum['fid']}" />
Replace with:
<div class="{$lightbulb['folder']} ajax_mark_read {$forum_url}" id="mark_read_{$forum['fid']}"></div>
Head to your forum homepage, right click on the icon of the forum you want to change. on the bottom it should say something like "
[email protected]_read_11.ajax_mark_read.forum-announcements"
Now go to: Admin > themes > your theme > global.css
Add to the bottom:
.on, .off, .off-lock {width: 22px; height: 22px; }
.on { background: url(images/on.gif) no-repeat; }
.off { background: url(images/off.gif) no-repeat; }
.off-lock { background: url(images/off_lock.gif) no-repeat; }
.forum-announcements { background: url(path/to/your/img) no-repeat; }
Replace forum-announcements with the name we got earlier.
You can do this for as many forums as you'd like and is much easier/efficient then using a plugin to do it.
This is based off of the default theme so the 22px may need to be changed accordingly.