Normally, when you press the Home and the Power button simultaneously on your iOS device, a screenshot of your whole screen is taken right away. Unfortunately, there's no way that you can take a screenshot of a particular portion area of your screen other than cropping it manually. This is why a new jailbreak tweak known as CroppingScreen debuted on Cydia recently to make your work a whole lot easier.
CroppingScreen: take screenshots of specific areas of your iPhone’s screen