This /is/ one-way by virtue of throwing information away: you can't rebuild the original image. If you're thinking of preimage resistance (it must be hard to fi

This /is/ one-way by virtue of throwing information away: you can't rebuild the original image. If you're thinking of preimage resistance (it must be hard to find an input that hashes to a given output) that's a property of cryptographic hashes, but not hashes in general (canonical example FNV).