Julius von Bismarck's Image Fulgurator is a sure way to mess with peoples photos at any popular tourist spot. Using a flash-triggered sensor, a slide with your message is projected onto the surface of the target area at the exact time an unsuspecting tourist takes a photo. Providing they use a flash of course. The result is a photo with a secret message embedded in it that wasn't visible to the photographer when the shot was taken.
There's a video demonstrating the Fulgurator in action but it's not so great, we'd love to see some actual photo samples on the site, the idea is awesome. The project just picked up a 2008 Golden Nica award at ARS Electronica in the Interactive Art category and a patent for the device has been submitted.