(Hidden in OS X Mountain Lion)
OS X Mountain Lion has a secret directory with 44 gorgeous high-res wallpapers including some truly amazing aerial photography and gorgeous scenery from National Geographic and the Hubble Space Telescope.
Each image is a whopping 3200×2000 resolution. To uncover these hidden wallpapers, press Cmd + Shift + G from your desktop to bring up the "Go to folder" dialog.
Then paste this in and press enter:
Now you can copy the four directories “1-National Geographic”, “2-Aerial”, “3-Cosmos”, “4-Nature Patterns” to where you keep your other wallpaper images and set them as your wallpaper.
You can also open your "System Preferences", go to "Desktop & Screen Saver" and add each directory by pressing the + button: