layer the effect over the desktop instead of taking a snapshot of it. That way if you like the effect you can continue working with the flames growing more and more.

Just like other have said have the effect start in like the corner icons and leap from one icon to the next until the entire screen is ablaze. And make it more of a desktop instead of a screensaver, or at least add the desktop as an option that way if you would like to continue working without having to reload it every time you can.