I've done this with friends, and in classes a long time ago. The idea is to begin the story based on the picture. The first person to write can start off the story however they wish to (any genre, any character names they come up with, etc.); no pun intended, but it's an open book. The second person to write continues the story from there, and so on. Feel free to go off the deep end and flip the script, or stick with the original storyline. It's a lot of fun, but maybe that's just the inner nerd in me talking