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

MAC (OS 9.x) Wallpaper Instructions:
Drag the image on to your desktop.
Go to Apple menu > Control Panel and select "Appearance".
Select "Place picture" and locate the appropriate wallpaper.
Click "Set Desktop".

MAC (OS X) Wallpaper Instructions:
Drag the wallpaper onto your desktop.
Go to Apple menu > System Preferences > Personal; select "Desktop".
From the "Collection" pulldown menu, go to "Choose Folder" and select the desired wallpaper.