PC Instructions
Click the appropriate link for your screen resolution and the photo will appear in your entire browser window.
Right click on the photo, then choose Set as Background or Set as Wallpaper (whichever option is available.)

Mac Instructions
Click the appropriate link for your resolution size.
Control click on the image and select 'Save Image As' or 'Download Image to Disk' (depending on your browser's options.)
Choose a location and click 'Save,' then open your System Preferences and click 'Desktop & Screen Saver.' On the Desktop page, click 'Choose Folder' and navigate to the save image file. Select it and click 'Choose.'