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.

Keys could be stored in something like TPM on Camera, and could sign the image. The key could then be verified from the camera itself to prove the authenticity of the image.

If we as a society deemed it necessary, the camera manufacturer could also provide a list of keys for devices they have manufactured. And an image/key could be provided, and the manufacturer could verify the authenticity that way.

The TPM signing could be tied into the sensor hardware itself, making it difficult, but not impossible, to sign arbitrary images with the TPM.

Or, you know, using the totally-real-picture camera to take a photo of an AI-generated scene?

Asymetric keys