Note: it is possible to blur images without affecting the rest of the file, due to the structure of JPEG:
http://mapillary.trydiscourse.com/t/mapillary-joins-facebook/4163/98?u=enteq
Additionally, the “diff” file could be a normal jpeg, with just everything white or black except the blurred parts.
And, if it one encrypts it with the uploader’s public key, it could even be stored safely on facebook’s servers. But I digress…