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