Url2png.Com is powerful screenshot service that allows you to grab screenshot of web pages by using their URLs. Easiest way to check this service is to visit its home page (url2png.com) type the URL and press ENTER. You will be seeing the screenshot image instantly displayed on the left, you can open that image in a new tab and save it to your hard disk. You can use this service in your website using its API key or simply try using its plugin if you use WordPress. Read its quick start documentation here.