Automatic website screenshots for Web app?

Is there a way to take automatic screenshots of a web page (by specifying its URL) in a web app using PHP or Ruby on Rails? Perhaps using a plugin or some external REST service.

I've researched a lot, and nothing seems to fit except something like this (http://www.binarymoon.co.uk/2010/02/automated-take-screenshots-website-free/) but I doubt Wordpress would just let me spam their servers for something not Wordpress related.

I'd like to use it for my own web application so I'm not sure of the legal implica开发者_运维技巧tions..how hard is this to implement myself? What does it entail?

Thanks!

Maybe this will be what you are looking for: http://www.webshotspro.com/ (it's not free but quite cheap)