One feature early iPhone adopters missed was the ability to take screen pictures. Apple acknowledged this lack, and they released the feature with the iPhone OS 2.0 release. Once you take a screen picture, it behaves just like any other photo you take with the iPhone's camera: you can use the picture on your computer, send it to someone else or save it with your other pictures in iPhoto.
Hold down the "Home" button.
Press the "Sleep/Wake" button at the top of your iPhone. The screen will flash.
Release both buttons.
Open your camera roll to find your screen capture.