The only "problem" with using the gallery is that some of those are no longer on the forum or don't participate regularly if at all. I can't see promoting someone who doesn't bother hanging around here. And it is a promotion. We had something like this in the weekly challenge --never got a response from the winner. IMHO
Hi Helene. Well if you did use the gallery uploads for the photo of the week, then presumably the user is at least active during that particular week!
I'm assuming that the POTW selections would be done on a timely basis so for example the POTW winner for the week ending June 26 would be announced on June 27. Yes there is a chance that someone will just post a picture during a given week and never come back again but that's probably not a huge concern. Most likely POTW winners will still be active a few days after posting a picture in the gallery.
vbulletin does have an awards system (not sure if you folks have activated it here) so you could create POTW and POTM awards and once given to a user, a special icon would appear in their user profile. Also, a quick PM should be sent to the winner to alert them to their award which would go a long way in avoiding the other situation Helene described - a winner not returning to at least acknowledge their award.
You also have to consider who will be assigned the task of selecting the winner who will then in turn make the announcement, describe why the image was chosen, PM the winner, etc.
Anyway, just my .02. Curt, your idea is a very good one!