Presumably the OP is proposing something like a TPM attached to the image sensor that signs the sensor output or something like that. You can’t sign it because you can’t get the key out. The key could be per-camera and be a published list.
I suppose a dedicated fraudster could still stage an appropriate scene. An appropriately lit matte image might even suffice.
That's assuming they don't just have a backdoor inserted expressly for this purpose. Now only the rich or powerful can produce an "authentic" recording of an event and the same system can be used to hunt down whistleblowers and political enemies by looking up who bought the camera.
At the very least there is always the analog hole where you point the camera at a sufficiently high quality screen. The sensor can't tell whether the light it receives was emitted by sunlight reflecting (and refracting) from someone's face or by a bunch of LEDs pointed at the sensor.
I suppose a dedicated fraudster could still stage an appropriate scene. An appropriately lit matte image might even suffice.