Pictures are at the heart of how we communicate with computers, emblematic of our cur rent fascination with multimedia and web-based computing. Nevertheless, most of us know far less about pictures and the way in which they work than we know about the text that often accompanies them. In an attempt to understand pictures, perhaps the most fundamental question we can ask is, "What is a picture?" What is it that objects as di verse as icons, bar charts, paintings, and photographs have in common that makes us refer to all of them as pictures? And what is it about pictures that convinces us to...