Overtime you may have collected a excellent resource of desktop wallpapers, however with the default windows settings you can only apply one wallpaper at a time. So even though you may have several good wallpapers you will not be able to make use of them unless and until you keep changing them at regular intervals.
WPTimer is a free portable software that will automatically change the desktop background using a selection of wallpapers you specified after specified intervals.
WPTimer or Wallpaper Timer allows you to choose one or all the wallpapers available and rotate them at a given interval. WPTimer will rotate the wallpapers available in the default wallpaper directory, which is usually %SystemRoot%WebWallpaper so you can copy and paste your own wallpapers to the default directory or read the guide to learn how to change the default wallpaper directory in Windows.