The manipulation of an image to alter its perceived editorial content is as old as photography. The problem today is it is so easy to do. Anyone with a computer and a small amount of photoshop smarts can edit people or objects in and out of scenes.
The flip side to that I think as more photographers or "journalists" try to get away with it, the more they will be caught and the more the public will view all images with a healthy amount of skepticisim as to the validity of the content.
I think the public is pretty sophisticated and understands that you must be very carefull regarding any content, print or image.
I remember back in the 80s when NBC news revealed that it used pyrotechnic devices the help support a story of Chevrolet pickup trucks being prone to catching fire in a certain situation. After learning that I have have had zero trust in investigative stories by a news organization. I think people will simply extend that level of trust to photographs as well.