Yes, but when you admit that the target is just the dumb criminals, then why adopt a scheme that has false positives? Decompress and downsample. Drop the least

Yes, but when you admit that the target is just the dumb criminals, then why adopt a scheme that has false positives? Decompress and downsample. Drop the least significant bit or two, maybe do it in the dct domain instead. SHA256. It'll preserve matching for at least some cases of recompression and downsampling. But finding an unrelated image that matches is as hard as attacking SHA256, the only false positives that could be found would be from erroneous database entries.