[PeterisP]

Assuming that media and consumers will want to consider photos/videos of random everyday people, it would require that:

1. All manufacturers, including manufacturers of shoddy but cheap mass-market devices (ones that a not-wealthy person would have on them to document interesting events) support that cryptographic signing in all their devices;

2. None of the signing keys/secrets can be ever extracted from any such devices;

3. None of these manufacturers or their employees ever generate a valid key (or a million valid keys) that would have been put in a camera of the same model that respected journalists use, but are just available to the government where the factory resides, or just for sale on some internet forum to sign whatever misinformation a resourceful agent wants to publish.

Signing pictures can mostly work with respect to a limited set of secure, trusted hardware manufactured and delivered with a trusted chain of supply, where a single organization is in charge of the keys used and the set of keys is small enough to control properly. E.g. Reuters might use it to certify photos taken by Reuters people using specific Reuters-controlled camera hardware (and they can do that just by ordinary signing of what they publish). But there's no motivation for most people in the world to accept that overhead for the devices they use for photography and video, and there's no single authority to control the keys that everybody else would trust due to international relations.