Windows 7: If you like to switch up your wallpaper often but are too lazy to go through the desktop background personalisation settings, here's a quick way to set a bunch of wallpapers at once.
Just as you can right-click on an image and set it as the desktop background, you can select multiple images (by holding down the Shift key or Ctrl key while clicking on the images) and select "Set as desktop background". The wallpaper will automatically rotate through those images at some set time interval (in my testing, the wallpapers rotated about once a day, but your mileage may vary. If you want the images to change at a different time interval, go to the desktop background settings where it says slideshow). Thanks for the tip, Abhijeet!