PC Download Instructions :
Right-click on the image and choose "Set As Wallpaper".

MAC OS9 Download Instructions :
Control-click and select "Save this Image as" to your Mac.
Go to Apple menu > Control Panel and select "Appearance".
Select "Place picture" and locate the appropriate wallpaper.
Click "Set Desktop"

MAC OS X Download Instructions :
Save image to desktop.
Go to Apple menu > System Preferences > Personal section and select "Desktop".
On the "Collection" pulldown menu, go to "Choose Folder" and locate the appropriate wallpaper.