so this would only make sense if you provide the original file for verification purposes

Yes, that’s exactly what I’m imagining. You’re keeping receipts for after-the-fact proof, in case it needs to be audited. If you have a newsworthy photograph, or evidence that needs to be presented to the court system, this could provide an important method of proving an untampered original.

Maybe a central trusted authority can verify the signatures and generate a thumbnail for verification (take the signed photo and put it through an established, open source, destructive algorithm to punch out a 200x300 lossy compressed jpeg that at least confirms that the approximate photo was taken at that time and place, but without sufficient resolution/bit depth to compete with the original author on postprocessing.