Nope, there's no such rule. Nor should there be, in my opinion. Yes, it does make it impossible to effectively police this group unless the admins keep a 24 hour vigil, but part of the reason for putting something here is so it gets favorited a bit. I guess you could set an upper limit on the number of favorites, but it's still a pain to remove things as they hit these arbitrary limits.
I go through the pool looking for interesting pictures that I might fave. I think that's the point of the group. If you add pics here in order for others to see them and possibly like them enough to fave, maybe you should visit the other members' pics in the pool, too.