It also occurs to me that the camera chips -- or even separately-sold chips -- could be augmented to perform transformations (like black-out) on already-signed images. You could even make this work with arbitrary transformations - just sign the new image along with a description (e.g., bytecode) of the sequence of transformations applied to it so far. This would let you post-process authentic images while maintaining authenticity.
The possibilities are pretty endless here.