JMaster
I saw this a couple of years ago on a Mac. It was wallpaper of the Golden Gate bridge, and during the day it was a day shot, and then in the evening it was a dusk shot, and after 8:00pm or so it turned to a night shot.
I thought that was really cool and searched around for something on the PC, but couldn't come up with anything. There are auto wallpaper changers (as already suggested here) but these are more suited to choosing new wallpaper randomly from a bunch of images. I'm sure that these programs could be made to do what you want, but there's got to be a program by now that is designed expressly for that function, and includes sample images, etc.
I'll keep looking, but hopefully someone else on here has some info!
l2c