The point is that an image of consequence, if real, would have a signed raw copy available which people could check to verify that there was in fact a real image that showed what a given edited image purported to show. If an image is being circulated but no one as been able to verify that a signed version exists, that image remains untrustworthy. This wouldn't stop people from editing images for aesthetic value.

I guess there'd have to be certified editors, which would be trusted to allow only "safe" operations on images (crop, white balance, levels, etc) and would sign the resulting jpegs themselves alongside the signature from the camera.

It doesn't really seem likely at all to work in practice.