If your Mac is password-protected, the first screen you see after start-up is the login screen which, depending on your OS, is usually a generic image chosen by Apple.
Loginox is a simple application that lets you easily customize the login screen. Simply drag and drop a new login image and Loginox will change it in a flash. It resizes the image automatically to a default of 2560 x 1600, with a ratio of 8/5. When you're happy, click on 'set new login image' for the image to take effect. You can switch back to your old image by clicking 'Reset'. Unfortunately however, Loginox doesn't seem to work properly with Snow Leopard and an update will probably be issued shortly to deal with this.
Loginox is a simple but effective application to customize the image that accompanies logins.