I make a point with Chester of trying to only upload photos of changes or new arrivals - there are, as you say, plenty of photos of the rest of the zoo!
I post any good photos I have taken, regardless of how often it is already seen in the gallery. The gallerys are a place where zoochatters can judge exhibits, build up an opinion of a zoo etc, yet this is not the sole reason I use it. I use the gallery to share my photographs with other zoochatters. In my opinion that is what the gallery is for, sharing photographs with people of similar interests.
If I took a stunning picture of, say, a rhino or lion at Chester, I would want to share it on here. What would be wrong with that? Yes, there are plenty of photos but I love looking through them all. It is certainly the richest photo collection I have ever seen
Sidenote: Maybe a seperate gallery could be set up for new arrivals/changes? That way we could keep the general gallery, and still have a place where people that want to judge new exhibits can search through quickly (maybe if that was read only and a moderater added photos?)