These illusions require a grid to activate them. To explain how these illusions work, you'll need to imagine that the images are made out of lines. The grid picks out every sixth line and displays it. When it's moved, it will jump to the next lines, showing a different picture.
These are all guaranteed to be PD, since they were made in Photoshop (and Google SketchUp) using self-made figures or PD images. It should also be added that the 3D models only are represented in a 2D format with shapes of (what I believe is) simple geometry. A simple shape is not eligible for copyright protection.