The logic of this belief about the relation of digital manipulation to traditional manipulation I think is a bit circular. With the development of Photoshop it became easier to manipulate images and manual retouching skills have declined, but when manual retouching was in wide use, there were lots of people around with the skill to change images in a substantial way that would be difficult for laypersons to detect. Perhaps the decline of traditional retouching techniques increases the "truth effect" of the traditional photograph, but at the same time, makes traditional photographs all the more subject to manipulation, since people are more likely to trust them.