Sterograms: In addition to containing unique works of art, these gallery images are stereograms; stereographic 3D pictures that are viewed by slightly crossing your eyes.
Viewing stereograms can be a little bit of a challenge to begin with, but the eye and the mind quickly adapt and seeing the image in depth becomes easier after a few viewings.
Click on the art to get a larger preview, then hold your finger up in front of the picture, just under the center of the image. Move your finger toward and away from the picture while focusing on your fingertip (keep your fingertip centered under the image). At some distance, (for the large previews here - about 14" in front of your face), you will see the left and right image beyond your fingertip merge to a central third image. Shift your vision between your fingertip and the picture while maintaining your focus on your fingertip. The picture will come into sharp focus, and you will see the 3-D image.
An alternative method is to hold your hands up flat with a space of about two inches in between them. Look at the space between your hands and focus on that area. Move your hands so that the image centers itself in between them. Then move your hands closer to the image or farther away until the picture comes into focus (about 14" for the large preview image). This technique tends to hide the two side images, and makes the cross-eyed center image easier to discern.
I prefer the first method, although I can simply look at the artwork, cross my eyes a little, and see the 3D.
When the 3D is in focus, you are actually seeing the left part of the picture with your right eye, and the right part with your left eye. The blended iamge can give an amazing impression of depth.
It may take a little practice, but the results are well worth the effort. It gets very easy to simply look at the picture and see the 3D or not.
A small portion of the population cannot see the 3D image within the stereoogram, because of various vision issues, but ~95% can view them with a little practice.