Steem is not built as file manager, and sharing keys would COMPROMISE platform seriously. There is no "Reading Hash Password".
Public - Posts are meant for everyone to see. Privacy comes to each user individually not between each other in blockchain.
Or at least you should explain in more technical keys, because abrakadabra private key/post private album/share is just a magic, not a technical possibility of API yet.
Do you know what are file permissions on hosting ? How you will manage permissions with encryption if you not encrypt entire file (then it's not image anymore and requires decoding to not be encrypter (base64) which is consider serious security flaw.
Magento CMS because of it has hacks all the year and constantly needs to place some security patches/leaks.