People sharing images that an authoritarian government considers banned might still be exposed by such a scheme, given they are likely to be exactly the same data. There are, after all, no new photos of tank man being photographed, any that are shared would be identical to someone elses, unless every recipient opened them up and modified them, and even then I'm not sure that actually modifies the data if done on an iOS device, as modifications done to images can be undone suggesting to me they are only a layer on top of the unchanged image, which would still return the same hash.

Unfortunately, I think the privacy problems surrounding iCloud Photos remain to an extent.