Select an image resolution, wait for image to load, right-click and select "Set as Wallpaper"

1280x1024 1024x768 800x600	1280x1024 1024x768 800x600	1024x768 800x600 640x480
1024x768 800x600 640x480	1024x768 800x600 640x480	1024x768 800x600 640x480