A cognitive illusion is usually a picture that is meant to show an ambiguous image or images. These images can be meant to confuse the senses or to require the mind to refocus attention to see both images. These kinds of illusions are categorized as ambiguous, distorting, or paradox illusions. Ambiguous images are figures that force the brain to switch back and forth between two separate images.
An example of this is the Necker cube. http://www.youramazingbrain.org/supersenses/necker.htm Distorting images are the images that use color and shape to give an impression of motion.
https://en.wikipedia.org/wiki/Optical_illusion#/media/File:Caf%C3%A9_wall.svg Paradox illusions are images that seem to represent the impossible. https://en.wikipedia.org/wiki/Impossible_object#/media/File:Poiuyt.svg