“When Mountain Lion was released, a bunch of fantastic wallpapers were hidden away and only used for screen savers,” Lifehacker reports. “It turns out Apple hid those wallpapers in a different place this time with Mavericks, but users over at StackExchange have managed to find them again.”
Lifehacker reports, “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/'”
More details in the full article here.