I've had the same background on my machine for almost a year now. A south atlas (south up), pacific-centered, night image of our lovely planet that wonderfully confused those that hovered over my shoulder and was an appropriate homage to my cartography profession. It was time for a change and it seemed simple enough. Found a good image and right clicked, set as desktop background. Suddenly my image wasn't stretched across two screens but instead duplicated on each screen. Even switching back to my old image produced the same result. No longer was South America inverted on the left screen and Africa in the right. Now the image was small and duplicated. What the heck? Way too many google searches later I finally learned that right-clicking on the image and then setting properties in Vista has different results than first opening desktop properties and then browsing to the image and setting the properties.
One Idea A Day, What I Learned Today