Desktop background

Somehow we lost the desktop background we had set. Went through the process
of selecting a photo background for the desktop and none of the photos that
came with the windows vista program will apply. Neither will any of our own
photos. The only background that will show up are the plain color settings.
The photos are available in the selection window but they won't apply. How
do we get the photo with the black and white leaf to show up again on the
desktop? Please help.