This is simply an LED light aimed at a light sensor. In our exhibit, both were at foot level- the light sensors under the table that held the paintings and the LEDs under a row of chairs several feet away. When someone breaks the beam by being between the light and the sensor, the light sensor notices a change in brightness, which triggers the painting.
The LEDs we used we soldered together ourselves. Each one had to be attached to a resistor in order for it not to burn out. Once the light was soldered to a resistor, it needed to be attatched to a longer wire in order to reach several feet away from the battery source. It took forever. But we kind of felt like Vulcan. Which was neat. We mounted the lights in LEGOs to make them easier to position. We used a breadboard to plug in all of our lights in series with a battery pack (far left red+black plug). Here is the fruit of our labors:
The sensors themselves came from PicoCricket and Scratch kits. Each had to be covered on top with cardboard to shield it from ambient light. Without the cardboard, the break-beam sensors responded to any change in the lighting conditions, such as shadow, or natural changes in sunlight, making them ineffective sensors.