Doubtless, you’ve seen it dozens of times: a still photograph within a video that somehow manages to appear three-dimensional. It displays depth, varying planes of motion, and perhaps even animated particles or lighting effects.

http://blog.visual.ly/the-2-5d-effect-how-to-animate-photos-and-create-a-parallax-shift/

This effect is often used in documentary films when they do not have film of an event, but only still-frames. Using the effect makes the image more interesting, and makes it seems like someone was there with a video camera and was panning back and forth.

from http://cgi.tutsplus.com/tutorials/create-a-realistic-camera-move-with-parallax--ae-453