After the release of Ice Cream Sandwich update for the Android operating system, screen capturing or running app image capturing is too easy. Either you have a Smartphone or Tablet which is running on Android Ice Cream Sandwich or higher version, you can easily capture the screen.
Google Nexus 5, 4 and Kindle Fire use Power and Volume Down buttons.
After capturing the screenshot, open the gallery. You will get a folder named as Screenshots under the Pictures. Any screenshot you take will be placed in this album.
For the devices, which does not supports screenshots, you can use a freeware tool from from Google Play store, like Screen Capture Shortcut Free or any another tool which supports your device. Currently, Screen Capture Shortcut Free supports Galaxy S, Galaxy Tab, and Galaxy S Ⅱ only.