Agreed, there needs to be trust, and how this used to work was that the photographer would gain a reputation for their accurate portrayal and would be picked up by an agency (e.g. magnum). They would supply the picture editors for the newspapers, so the public would have a chain of trust back from the the paper to the photographer.

The equivalent for digital photography is that cameras can sign images - I believe there are some canon/nikon cameras which supported this to allow digital photos to be used in court, but I don't think this idea was widely adopted, so a typical snap on your phone doesn't provide any sort of trust, which is surprising really, but there we are, this is the world we live in.