I don't think that the signer would be able to verify the authenticity of the data that it received from the sensor and image processing circuitry unless they were able to authenticate each other securely. I know that an attack on a system like you proposed would still be expensive, but it would become more attractive if its characteristics were overplayed (and would then be subject to legal challenge). Forensics, of course on the other hand is based on experts saying "yes, by all accounts this appears to have happened".