How to change images of these

goto ACP >> Templates >> Forumbit templates >> forumbit_depth2_forum

and find....
<td class="{$bgcolor}" align="center" valign="top" width="1"><div class="forum_status forum_{$lightbulb['folder']} ajax_mark_read" title="{$lightbulb['altonoff']}" id="mark_read_{$forum['fid']}"></div></td>

replace it with....
<td class="{$bgcolor}" align="center" valign="top" width="1"><img src="images/ficons/{$forum['fid']}.png" /></td>

Now goto, Cpanel >> File Manager >> Your Forum Path >> images >> create a folder named "ficons" and upload the icon images inside it named after the forum id

so for a forum whose id is 4, you need to upload a file named 4.png inside /images/ficons folder...

This is what I found online.