Select the Gallery tab above. Click on Wallpapers. there's a lot to choose from.
to use:
Right click on the desktop, choose the tab called "Desktop", scroll through the list shown under background and choose a background image. You can also browse to another directory on your computer where you may have other .bmp or .jpg files and choose one of them. On the right side there are choices (position) which effect how the picture is displayed. If the picture is smaller than your display you can choose stretch to fill the screen or tile to make multiple copies show up.
Alternately, you can use Windows Explorer to find pictures and then open them with Windows Picture and Fax Viewer. Right click on the picture and choose "Set as Desktop Background"