Mavericks has a fine array of background images, forty-three 3200-by-2000-pixel photos to be precise that are hidden away from the prying eye, unless you know where to look.

In Mountain Lion they were stuffed away in
/System/Library/Frameworks/ ScreenSaver.Framework/Versions/A/Resources/Default Collections/,

but in Mavericks they’re in

/Library/Screen Savers/Default Collections/.
Note that the new location is not the user-level ~/Library, but simply the filesystem root-level /Library.

wpid 602039316 How To Find Those Wallpapers In OS X Mavericks

But if you’d prefer to browse through the 43 images and choose one or more for your desktop background, go to the Finder, press Shift+Command+G, in the Go to Folder dialog that appears type /Library/Screen Savers/Default Collections/, peruse the images at you leisure, and copy any that you find fetching into a convenient folder.
Then launch System Preferences, choose Desktop & Screen Saver, click on the Desktop tab, click the “+” sign in the lower left, navigate to the location into which you moved the images, and knock yourself out.