What might be great here is to combine points and badges with the feature. So if the user has x points then he got the badge cover designer or something and can then change his profile cover. This might prevent the system from adding background pictures which aren´t that "nice". The user must then be part from the community longer then x days to get x points until he can do that.

This would be a kind of Gamification feature ... if the user whish to perform that he need to be an active part from the community. It would also prevent that 500 users uploaded 500 x 300 KB cover pictures which was then x GB of space and they will never come back to the community.

P.S. related to the following postinghere.