CSS Animation ability?

Hi All, I'm working on a project to combine what I've learned so far with treehouse and I have an idea for it that I'm not 100% sure is possible. I remember in one of the css videos (in the animation part I'm pretty sure) Guil put two images into the same place one acting as the front and one acting as the back. I'd like to use an if/else statement and Math.random to use a similar animation but randomize the side of the picture that shows; only I can't remember if the animation had the ability to show the back image instead of the front image or if it only displayed while the animation was moving. Does anyone remember which video this was in so I can review it? Does anyone know if this would even work? For what its worth my plan is something like if Math.random = >.5 show side a and if <.5 show side b. Thanks in advance for any help/suggestions.