If you're using a Mac with a built-in iSight camera, use the built-in iSight camera to take a single snapshot of yourself, a 4-up snapshot (that's four quick shots in a row), or a movie clip.
Photo Booth places thumbnails in the drawer at the bottom of the window.
Ready to set your iChat buddy icon?
Photo Booth copies the image, quickly opens iChat, and displays the photo in the Buddy Picture window. Use the slider to resize it if you'd like. Click Set, and you're done.