Please read the description..

Navigate to /Library/Themes/ or, if you can't manage to find this folder, go to /var/stash/Themes.XXXXX/ and then to "LS Climacons". In there are three wallpapers to chose from, but you can also put your own in there. Just name the wallpaper you want to be used "LockBackground" (without the quotation marks of course). The file extension must be .png