How to enable revisioning for user photo (user/profile field)?
The User Revision module would probably be your best bet

Add revision support to users and fields like there is for nodes.
For the moment all field from {users} have revisions, except for password

The picture property is contained in the users table so this module should do exactly what you're looking for.
The issue of a UI for accepting or declining a photo should go in another question otherwise this one will become too localised.