Couldn't you just take a picture with the same camera and then hash it? The probability of getting the exact same set of pixels is small and it's certainly not predictable.
I wondered the same thing. Noise in the picture should also make it extremely hard to predict. For cryptographic use one would only need 256 bits of entropy to seed a CPRNG, and I can't see how this amount of entropy wouldn't be present in a normal photo.
