I think hashing pictures and storing the results to compare with future uploads is a perfectly reasonable thing to do. This is the same thing that is done with CP pictures and for much the same reason.
The only thing that makes me nervous is the actual uploading of pictures in order to be scanned. It should be possible to create a client-side image processor that generates the appropriate hash but doesn't involve uploading your pictures to a server somewhere. Perhaps a Javascript processor in-browser or even a native desktop app.
