It's signed, not encrypted. Think of it as a chain of custody mark. The original photo was signed by person X, and then edited by news source Y. The validity of that chain can be verified, and the reliability judged based on that.

Effectively it ties the veracity and accuracy of the photo to a few given parties. E.g. a photo from a known good war photographer, edited under the "New Your Times" newspaper's licence would carry a lot more weight than a random unsigned photo found online, or one published by a random online rag print.

You can break the chain, but not fake the chain.