A spectator scrolls through photos on your iPod touch or iPhone and chooses one at random. You are then able to tell them what photo they have chosen.
This can be performer on any phone which is touchscreen and is similar to the Iphone (E.g Samsung Galaxy).
No special app, no locking the iphone, no calibration, no duplicate photos, no menus, they can stop wherever they want, a spectator is the one that scrolls through.
You can adapt this trick to perform it with pictures of anything.