As first pointed out by OS X Daily, Mountain Lion's built-in screen savers cycle through a couple dozen awesome photos, across several categories, with images of space, nature, and more. The images are huge—and thus make great desktop backgrounds.
To find the images, use the Go To Folder command in the Finder (from the Go menu) and navigate here:
There, you'll find four folders full of high-resolution imagery. You can copy them elsewhere, or even access the images from right where they are. Simply fire up the Desktop & Screen Saver preference pane, click the Plus (+) icon at the left, and add the folders—or individual images—as desired.
Mac OS X Hints