^ you should probably close your <a href> tag at the end before the </if> tag.
Code:
<!-- Site Supporter Code -->
<br />
<if condition="$post[usergroupid] == 2 and is_member_of($post, 20)"><a href="https://www.paypal.com/cgi-bin/webscr"><img src="images/misc/supporter.gif"></a></if>
<!-- / Site Supporter Code -->
btw, good idea. but will it link the user to YOUR website's paypal contribution page?
I just may add your contribution to this thread's first post and give you credit for it.