This is a 3D image. It was created using techniques described here: [link]
To view a 3D image like this, you put your face about a foot from the screen, and hold your eyes exactly level with the image. It helps if you can view the image in fullscreen mode so there is nothing else on the screen to distract your eyes. Cross your eyes slightly and you'll see a "ghost" image appear between the two. Adjust how crossed you've got your eyes until the central image is correctly aligned and in sharp focus and it'll appear to be 3D!
Model is Melissa.
This image is not stock!