screencaptureutility to initiate a screenshot from the command line.
Take a screenshot and save the result to a file
This command can be used to take a screenshot with a 10 second delay (-T 10) and save the result in jpg format (-t jpg) to a file named screen.jpg:
screencapture -T 10 -t jpg screen.jpg
If the file format is not specified, a png file is saved. The time delay lets you change desktops if you are using Spaces (the multiple desktop feature in Leopard) before the screenshot is taken.