Originally Posted by Exoskeletal

Well it takes some practice. First of all save the image, open it and zoom in, so the whole image is on the screen. Then look at the center with your eyes crossed ( I put a finger betwen my eyes to help it, and when I have the third image I take the finger away ), then relax your eyes and focus on the center image.
Edit: And always try to focus on the backgournd.