Actually incorrect! []/forum/images/icons/grin.gif[/] The images don't have to be the same size. They do have to be square so as not to be distorted. The image is actually constrained to a size you specify with the height and width tags. Here is the code modified from what Dave posted above with the align and width height tags.
<img src="{$config['images']}/{$forum[$c][$f]['Keyword']}.jpg" height="64" width="64" align="left" 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>
Now what would be really nice since I do not know how to create a transparent image would be that if no gif existed that it would default to board normal.