Thum.io provides real-time screenshots of websites. It is a great website snapshot generator that streams screenshots as soon as you request them. There are also more advanced options which let you resize the image by specifying the width, as well as how many pixels of the original website you want to crop.
Fast: Thum.io streams the initial render as an animated GIF, so your users get content fast. Existing screenshot services make your users wait 10-20 seconds while they hit your URL, wait for it to load, take a screenshot, process it, and send it back to you. Thum.io responds immediately with a few frames of a loading spinner.
Secure: Every app is different, so Thum.io offers many options to secure the requests for your account. If you want something simple, you can authenticate by referer. If you want something more secure, you can accept signed requests that have a custom expiry time.