In the other thread you referenced, it looks like this is what calls the icon image....
Code:
<td valign="top" width="70" align="center">
<a class="alt" href="home.php?cat={ $subcategories[cat_num].categoryid }">
[img]../icon.php?categoryid={$subcategories[cat_num].categoryid}[/img]
</a>
</td>
<td align=left><a class="alt" href="home.php?cat={ $subcategories[cat_num].categoryid }">
{$subcategories[cat_num].category|escape}</a>
BTW - it looks like that code may be corrupted, the [img] tags should probably be <img>.
Anyway, what if you added a class to your stylesheet named maybe imgicon and have border: 1px solid #000000; as the properties?
Maybe something like...
<a class="alt" href="home.php?cat={ $subcategories[cat_num].categoryid }"><font class="imgicon"><img src../icon.php?categoryid={$subcategories[cat_num].categoryid}>
</font></a>