Click on THE RESOLUTION SIZE of the wallpaper image you want (see the selection below).
When the image comes up in the browser window, click and hold on the image to bring up a pop-up menu, then choose to save the image to any convenient location on your hard drive. (Ex. System Folder > Appearance > Desktop Pictures.)
Under the Apple menu, go to Control Panels and select Appearance.
In your Appearance control panel, click the tab marked Desktop.
Click Place Picture (if you already have another picture as your desktop image, click Remove Picture first, then Place Picture). Use the dialog box to locate and open the image you downloaded.
Underneath the Place Picture box there is usually a pulldown menu on how you want the image placed, ie. center, tiled, or positioned "automatically." Select the one that fits your monitor screen size best.
Click Set Desktop and close the control panel.