> Personally, I would love to see a strategy for generating a private from an analog picture.. But I don't see a good way to do it reliably.. Maybe one could generate it from error correction bits in a QR code

Not clear exactly what you meant, but but deriving a key from a photograph shouldn't be that difficult. A good perceptual hash should work, maybe some preprocessing is needed (straighten/crop/normalize/resize). It helps that the amount of data in an image is absolutely massive compared to typical key size.