PC Download Instructions:
Right-click on the image and choose "Set As Wallpaper"
 
MAC OS X Download Instructions:
1. Control-click and select "Download image to Disk".
2. Save image to desktop.
3. Go to Apple menu > System Preferences > Personal section and select "Desktop".
4. On the "Collection" pulldown menu, go to "Choose Folder" and locate the appropriate wallpaper.