In my office we have a computer that displays our website on a big plasma display. It's basically Firefox with the plugin reload-every set to reload the page every 10 minutes.

My question is, for a complete newbie, how difficult would it be to make a program that took a screenshot of the website every ten minutes and display it (fullscreen)? Ideally I'd even like to make it so that when it switches from the old screen shot to the new one it would do a cross fade...

Its very very easy to reload a page every 10 minutes, you dont need any plugin. It can be done by the website itself to reload itself using javascript or meta tags. To display full screen, just press F11 on the browser.