rotating wallpaper?

I've heard you can make it so your wallpaper changes every hour or so, selected from a pool of images you choose. Does anyone know how to do this, or whether or not there's an app for it?

System Prefs>Desktop & Screensaver>Desktop>Select the album you want to use as the background>Near the bottom there should be a check box that says “Change picture” and then how ever often you want it to, check that box and select 1 Hour.

I have included a screen shot to show you where that box is.