Apart from the privacy issues, I guess the challenge would be how you preserve the signature through ordinary editing. You could embed the unedited, signed photo into the edited one, but you’d need new formats and it would make the files huge. Or maybe you could deposit the original to some public and unalterable storage using something like a blockchain, but it would bring large storage and processing requirements. Or you could have the editing software apply a digital signature to track the provenance of an edit, but then anyone could make a signed edit and it wouldn’t prove anything about the veracity of the photo’s content.