You'd need some way of generating an image key that you don't have access to and that their client can somehow use. I don't see how to securely generate a key on your device that is still secure on theirs. You'd only be able to do this with people you trust absolutely, who use reasonably secure devices. So that's a very small list for me at least.
Discussion
Encrypt to their public key
Encrypt what though? If you're encrypting a key then that means you had it in plaintext. Same security problem I've outlined. And they are still decrypting on a potentially insecure device, making rugging more likely if the tech were widespread.
Yes, the input of anything being encrypted is considered plaintext, even if it itself is encrypted.
I think we need a clearer definition of the problem and goal. Im interpreting as a user wanting to send value to another user they know and can identify by nostr pubkey, and have it renderable as an image (and thusly encoded within) for which only the sender or recipient could claim the value.
Sending a kind 1 note referencing a publicly retrievable image whose contents encode an encrypted ecash string isn't the most efficient but could be used for such
I agree. I don't see a problem nor goal. The answer to the question is yes, because it is possible. But that doesn't make it a good idea, which is my point. I just don't see how it does anything other than create more technical and security problems than current options.
All of these issues are why storing a seed phrase for example as an image on a phone is a dumb idea.