[For 1.8] Square theme

[Triage]

How can I add a custom Icon to one of the navigation tabs? I added a donation tab to the top and I want to know how I could possibly add an icon to it along with the text. Thanks

This is what I am using but the icon is not showing up

<li><a href="{$mybb->settings['bburl']}/donate.php"><img src="{$mybb->settings['bburl']}/{$theme['imgdir']}/square/dollar.png" alt="" title="" /><br/>Donate</a></li>