These are stereoscopic images that you view with your eyes slightly crossed.
To view one of these images, display it at it's largest "original" size. Then sit at a comfortable distance from your monitor and look at the dividing line between the images. Then hold your finger about halfway between your eyes and the screen so it appears just at the bottom of the image. Then look at your finger and slowly move it closer to your nose. This will cause you to cross your eyes. As your finger gets closer to your eyes, the left and right images will cross over and at some point you should see a stereoscopic view in your field of vision between the two images on the screen. It takes a little practice. Once you are able to see the stereo image, you should be able to hold your eyes in position and remove your finger.

If you have trouble seeing the stereoscopic effect you may need to move a little further from your monitor and try again.