Often, there are very good reasons for switching heads in photos. The photographer or client may like the smile on one pose, and the lighting and body position on another. Rather than re-shoot, the photo editor can combine the photos in Photoshop by replacing the head. Of course, this technique also works for creepy or funny effects such as a backward head on a forward body, the wrong heads on the wrong bodies or even the same head on everybody in a group photo. Warnings Writer Bio

How to Take the Head From One Picture   Put it on Another Photoshop  14 Steps  - 83