Using a procedure to scatter pixels in an image¹ it's possible to "encode" a second image onto the result, and have two discernible images contained within one set of pixels.
The process is simple. Scatter the pixels, then multiply (or add, etc.) the result with the image you wish to include. This effectively "stains" the pixels in that position, meaning that, encrypting and decrying the image will flip-flop between which image is displayed.
¹ see also: http://oioiiooixiii.blogspot.com/2014/03/a-novel-approach-to-encrypting-images.html
