Apple have a knack for picking and shipping some stunning wallpapers with their OS.
And it looks like OS X 10.8 Mountain Lion will be no exception judging by the 15 or so backgrounds present in the most recent developer build: -
If you like what you can (barely) see in the image above, and want to use them on Ubuntu, then the folks at OS X Daily have got you covered.
Despite there likely being legal issues associated with third-party distribution of the photographic marvels, OS X Daily have, nevertheless, put all 15 wallpapers up on their site for anyone to download.
Better yet they are in humongous 3200×2000 resolution.
Don’t let the fact that these are included by Apple out you off; they are licensed from a variety of photographers and companies and look wonderful with Ubuntu’s chameleonic Unity interface.