Apple Mac
Click on the desktop size you require, then right-click (or ctrl-click) the mouse button on the image that opens. From the contextual menu that appears choose 'Use Image as Desktop Picture'.
Or you can 'Save Image As...' then open System Preferences and click 'Desktop & Screen Saver'. Use the file directory on the left or click the '+' button to navigate to where you saved the image, and select it.