> You could prove the photo was real.

No. You'd only ever be able to show that key material belonging to $specific_camera was used to sign/mark the image.

Was the camera manufacturer breached? Did somebody on the factory floor steal some keys during the provisioning step? Or did somebody build their own photo-sensor simulator and plug _that_ in to the camera's motherboard to feed it a "real" image? Before going _that_ far, just point the unmodified camera at a sufficiently high resolution display...