When Mountain Lion was released, a bunch of fantastic wallpapers were hidden away and only used for screen savers. It turns out Apple hid those wallpapers in a different place this time with Mavericks, but users over at StackExchange managed to find them again.
These are the images that Apple uses in their screen savers, which means they're high resolution nature photos. To grab these, just press Command+Shift+G from your desktop to bring up the "Go to Folder" menu and type in:
/Library/Screen Savers/Default Collections/
That takes you to the new folder location where you can copy those high resolution images and use them for your wallpapers.