Daniel Hruska OS X Mountain Lion offers 44 nice high-resolution images (3200x2000 pix) used for the screensaver, but you can't get to them at first glance. However, nothing is impossible and we can access these photos only with the Finder.
So start the Finder, go to the menu Open > Open folder (keyboard lovers can use the combination ⇧⌘G) and enter the following path:
/System/Library/Frameworks/ScreenSaver.Framework/Versions/A/Resources/Default Collections/
The folder where the wallpapers are stored will open. By right-clicking on any of them and selecting Set as desktop image you simply set it as wallpaper. Of course, nothing prevents you from copying all the files to another folder so that you don't have to access them in such a painstaking way.
Note: Some readers may wonder why I have multiple tabs in one window in the Finder. This is an add-on TotalFinder, who can do even more.
Finder in 10.9 too..
You can also use Safari (I haven't tried other browsers). Just paste the copied path into the search bar and confirm.