Engage students in a tournament of “layer tennis” (also known as Photoshop ping-pong”. According to Wikipedia:
The players pick a starting image, or one is “served” by a player, then another player makes some sort of alteration to the image in any chosen image editor (matches are not exclusive to Adobe Photoshop). They then send the altered image to the other player or players, usually via e-mail or by posting the image to a Photoshop tennis forum, who then edits that image and sends it back to the first player. This process goes back and forth until a predetermined number of rounds have elapsed, or the players otherwise wish to end the game.