Click on the correct resolution for your monitor size (to check this, right click on your desktop and Select Properties > Settings. Your resolution can be found under 'Screen Area' in the bottom right of the Properties box).
You'll see a full size version of your wallpaper. Click on it with your right mouse button.
A Windows menu should pop up.
Click on 'Set as Wallpaper' or 'Set as Background'.
Your wallpaper should automatically appear on your desktop.
Click on the correct resolution for your monitor size (to check this, click on the apple and Select Control panel).
Choose 'Download Image to Disk' and save the image.
Open 'Appearance' in the control panel.
If you already have wallpaper set on your Mac, click on 'Remove Picture' and then 'Select Picture'.
Choose your Virgin.net wallpaper and click on 'Set Desktop'.