Mountain Lion has a semi-secret directory stuffed full of beautiful wallpapers, including photos from National Geographic and images taken with the Hubble Space Telescope. Luckily, they're easy to find and use. You just have to know where to look.
This tip comes to us from the folks at OS X Daily. 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:
If that folder path or the images look familiar, it's because OS X uses the high-resolution photos for the stock screensavers. Once you have the folder open, you can copy the images to where you keep your other wallpaper images, upload them to Dropbox or use them anywhere else you need fresh desktop wallpaper.