In the "default" directory of your "templates" directory find the ubbthreads.tmpl file.
Change these lines:
<font class="forumtitle">
<a href="{$config['phpurl']}/postlist.php?Cat=$Cat&Board={$forum[$c][$f]['Keyword']}">
{$forum[$c][$f]['Title']}
</a>
</font>
To this:
<img src="{$config['images']}/{$forum[$c][$f]['Keyword']}.gif" alt="{$forum[$c][$f]['Title']}" />
<font class="forumtitle">
<a href="{$config['phpurl']}/postlist.php?Cat=$Cat&Board={$forum[$c][$f]['Keyword']}">
{$forum[$c][$f]['Title']}
</a>
</font>
And then upload your board related images to your image directory. The board related images need to be named with the board's keyword. If you don't have a specific graphic to use with a board I sugges creating a 1 pixel transparent gif to be used so that there are no blanks displayed. []/forum/images/icons/smile.gif[/]
This is the easiest way of doing this hack. Hope this makes it easier to understand.
PS I used the .gif extention in this example. You can change that to .jpg or what ever but it will need to remain the same for all boards.