I don't know if this will work but try...
Code:
<if condition="is_member_of($userinfo, 10)">
<if condition="is_member_of($userinfo, 5) OR is_member_of($userinfo, 6) OR is_member_of($userinfo, 7)"><img src="http://www.tech-unity.com/forums/images/ranks/premium.jpg" alt="Premium Member" /></if>
</if>
another alternative would be to create another usergroup for Staff members who have 'premium' status