If you go into "System Preferences..."->"Desktop & Screen Saver"->Desktop, you'll see at the bottom that you can change the picture every, say, hour. I think this is what unixkid was referring to. Now, assuming that you pick a folder to choose the desktops from, and assuming that you enter 24 desktops, 6 each (identical images but with different names) of morning, afternoon, evening, and night, then this should work.
Not the cleanest solution. But a start, perhaps.