I use a secondary usergroup and this is my code for the postbit. It lets users enter the url to the pic in a custom profile field. You could add the same code more or less in any other template you like.
HTML Code:
<!-- begin sidebars -->
<if condition="is_member_of($bbuserinfo, 5, 6, 7, 50)">
<div>
<if condition="$post['field19'] != ''">
<img src="$post[field19]">
</if>
</div>
</if>
<!-- end sidebars -->