I recently discovered that Mountain Lion has a few folders of high resolution pictures that are normally unavailable to users. From what I hear, these are screen saver images for the Apple TV. (I haven’t verified that yet though.)
To find the images click on the Go menu and choose Go to folder, or press Command-Shift-G, and enter the following path:
/System/Library/Frameworks/ScreenSaver.Framework/Versions/A/Resources/Default Collections/
You’ll then have a folder with four subfolders in there: National Geographic, Aerial, Cosmos and Nature Patterns. You can now either copy those folders to your desktop, or drag them directly to the Desktops & Screen Saver preference pane to add them to your desktop possibilities.