Provided the signature of the original raw pixels is left in the final JPEG, I think Canon/Reuters can be expected to produce those original bits to any sufficiently curious/legally authorised third party, who can verify the signature to check that they are indeed the original bits. At that point said third party has the initial and final images, which is likely enough to conclude whether the edits were legit (the chain of edits themselves isn't as interesting).

Mostly agree on the blockchain shenanigans, though I suppose using an external blockchain allows you to prove that the image was taken before a certain point in time.