
You can configure the option for shuffle, time delays and how the image will fill up your desktop. Once you apply the desktop wallpaper will change automatically and take images from the folders you have added in the library.

Right click on Pictures folders in the Libraries > Click Properties.Now since Picture Library is supported in Desktop background, any folder or sub-folder can be accessed.

One of the folder My picture is where all your pictures get stored. In Libraries, there is a Pictures Folder which is the default place where you can configure to bring all your images for easy access. Also it has pre-configured folders like Top rated, Picture Library, Desktop Background. Windows 7 lets you browse and select any directory to change wallpapers. However Windows 7 Library can help you in including sub folder or any some folder you want to include in changing desktop background, also called as Wallpaper Slide Show.

Though Windows 7 came with changing wallpaper automatically but including lot of wallpapers from different folder or sub folder is little tricky as you cannot include more folders directly.
