I think most users of SMF forums is using membergroups based on Post Count. It's the easier way to "promote" members.
I pretend to promote the involvement of members on commenting each others photos and to restrict uploads that are starting to consume too much space on the server. The way I thought to make this together is to restrict the uploads via number of comments made. This way, more involvement means more uploads available.
It would be total number of uploads for all time by membergroup. Example: A member is new and has no comments, he can post up to 10 photos. After reaching 10 posts, he changes membergroup and the limit is upgraded to 25 and so on.