This is a stereo image. In order to experience the effect, you’ll need to cross your eyes till a third center image converges in the middle.
Sit back 24inches from the screen with the image as large as you can make it; full resolution. You must have both eyes open at all times, that’s what makes it stereo.
Look at the center of the side by side image . Slowly begin to cross your eyes until there appears to be three images before you. You will need to relax your eyes a bit, and be sure to keep your head level with the horizon of the two images. The image will seem to snap in place and create a real 3d sensation.
Also you could try placing your finger at the center of the two images, focus your eyes on your finger as you bring it slowly to the bridge of your nose. When it appears that to two images have converged into one behind your finger, pull your finger away and try and focus on the middle image.
What is also helpful is to block out your peripheral vision with your hands after you have focused on the center image. Place your finger tips on your forehead and slowly block your side vision with your palms. You can actually block out the side images with your hands and leave just one image to enjoy.
Don’t spend too long in this cross eyed world.