I think I understand what you want. Try this:
Code:
<!-- Site Supporter Code -->
<if condition="is_member_of($post, XX)"><img src="images/misc/supporter.gif"></if>
<!-- / Site Supporter Code -->
XX = additional usergroup ID#
as far as having to list every primary usergroup, that's the only other way I can think of to do it. maybe another coder will come in here and help.