Determine the image size appropriate for your monitor.
Click the size you wish to download.
After it appears in your browser, drag the image onto your desktop.
From the Apple Menu (at the top left of your screen) select CONTROL PANELS, then DESKTOP PICTURES or APPEARANCE. (If the new window reads APPEARANCE, select the tab that reads DESKTOP.)
If necessary, click REMOVE PICTURE to clear the current desktop picture.
Click SELECT PICTURE or PLACE PICTURE, then locate and click the image file.
Click OK or CHOOSE.
Finally, click SET DESKTOP.