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).

A hash is any function that produces a fixed size output. It’s not required to be one-way, or to even change the input. You might be thinking of a “good cryptographic” hash. For that, it’s desirable to be one-way and to scramble the input, but that’s a specific subset of hash functions.

Edit: to the downvoter, please have a look through https://en.wikipedia.org/wiki/Hash_function

Can you show me then how you decode this string to the original image?