The Google Desktop Sidebar has a little picture slideshow that shows random pictures from any where on your machine, or from folders you select. You can disallow certain folders, too, if you have images you'd rather not have showing up on your Desktop. I like it for the same reasons; it brings up images and entire vacations I hadn't thought of in a long time.

I'm sure other people have made random images screensavers, haven't they? It doesn't seem like a new idea.