How to Digitally Modify a Stock Photo
One of the things that drives me crazy is when a designer chooses a couple of stock photos that may (or may not) relate to the book, and superimposes one upon the other. Usually it just looks like a kooky double exposure to me.
This post on David Drummond | Covers is a thoughtful digital modification of a photo. In this case, it was an author recommended photo, not stock. But the same works for stock.