Browse through the several pages of wallpapers and pick a few you like. Currently all the wallpapers are sized to 1024 x 768 or 1024 x 683. When we get more up we will begin to size them for the larger monitors out there. I am currently using a 27" ViewSonic flatscreen (very nice!) so I know that these sizes are too small for the larger new monitors.
Click on the image to view the wallpaper. Then to make it your desktop, right click on the large image, select "Set as Background" or "Set as Desktop". In windows, the image might be stretched if you have a large monitor.
You can center the image to create a border: right click on the desktop, select "Customize" or "Personalize". Then select "Background" or "Desktop". From there you can choose the display options and a border if you like.