How to Take Online screenshot of a computer or website ?
Snag is an Awesome tool you can simply press PrntScr key on your Computer and paste it.
Give a Try to http://www.printscreenshot.com/
Here is Screenshot Tool http://prntscr.com/ For Mac and Windows.
Generate Full Screenshot or Thumbnail of a webpage with http://www.page2images.com/
Get More better https://screenshotmachine.com & http://ctrlq.org/screenshots/
Even check out this Chrome extension.