OS X Mountain Lion brought us the single feature we all care about the most: new wallpapers. There weren't that many new ones though, and worse, Apple actually removed most of the older wallpapers.
Turns out that Apple actually included dozens more of these Retina-quality images in Mountain Lion to be used in various screensavers. You can access all of these images through the Finder by following the steps outlined in this OS X Daily article to copy them to wherever you'd like and set them as your wallpaper. You new wallpaper.