Cryptographic hashes aren't supposed to have collisions... the types of hashing that PhotoDNA and other similar systems use are not cryptographic. In fact, som

Cryptographic hashes aren't supposed to have collisions... the types of hashing that PhotoDNA and other similar systems use are not cryptographic. In fact, some say they aren't even real hashes. The reason for this is because they want images that have been altered to still match, whereas with a cryptographic has, even a minor change will change the hash. So it is more a a "similarity" check than a proper hash.