Regardless of which model iPhone you own or its system version, you can capture an image of your phone's screen by pressing the "Home" and "Sleep/Wake" buttons -- no need to install any extra apps. You can take screenshots from inside any app or website, and they include everything currently on the screen, even active pop-ups such as the volume level.
Take a Screenshot
To capture the current screen on your iPhone, press and hold the "Sleep/Wake" button and immediately press the "Home" button. The screen flashes to indicate the snapshot, and the phone saves an image of the screen.
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Use a Screenshot
To see your screenshots, open the Photos app, tap "Albums" and tap "Recently Added" in iOS 8 or "Camera Roll" in iOS 7. This album contains both your photos and screenshots.
Tap on a screenshot's thumbnail to enlarge it and then press the "Share" icon -- a square with an up arrow -- to send the screenshot by text message or email or to upload it to a social media site. Press "Edit" to make minor adjustments to a screenshot, such as cropping the image.
To copy the picture to a Windows PC, connect your iPhone and computer with the USB connection cable, press "Windows-E" to see your connected devices, and open the iPhone to access its photos. Alternatively, email the image to yourself or upload it via a cloud storage app.