url to image: How to create a screenshot from an url?

[eluser]marcox[/eluser]
[quote author="garymardell" date="1247621997"]I'd look at using a site similar to http://www.thumbalizr.com they have an api and set servers to handle all the work, for better screenshots you need a "premium" account though.[/quote]

My main problem is that i have to create this screenshot depending on an user's input - just like thumbalizr. After creating the screenshot it has to be saved to a file. I work on a linux system with root access so if you have some more hints for me i can try some ways out.