Thomas Brand at Egg Freckles has written a short history of Apple desktop pictures through the years. "Mavericks' New Wave" talks about how the Apple desktop has evolved since Mac OS 8 and the quality of the desktop patterns and photos.
Thomas includes download links to copies of the desktop picture backgrounds used in different Mac OS X systems via his dropbox account. He thinks that the new OS X 10.9 Mavericks Wave desktop picture is the "best default desktop wallpaper the Mac has seen in a long time."
In his TUAW article, "Grab the OS X Mavericks wallpaper now," Michael Grothaus relates that the "beautiful image of a cresting wave [for] OS X 10.9 Mavericks is meant to represent the surfing spot that the OS is named after." Unfortunately, the link in that article to download the desktop no longer works, so please use Brand's link to his copy of the Mavericks Wave [direct download link].
On Another Note
For those of you who are still interested in enhancing your Mountain Lion Desktop, I found an article from 2012 on OSX Daily that may interest you. The article, titled "43 Amazingly Gorgeous Secret Wallpapers Hidden in OS X Mountain Lion," contains instructions for locating four folders that contain 3,200 × 2,000 resolution images you can use as your desktop pictures. These are the images used in Mountain Lion's screen savers, but can be used individually. Thumbnails of the available images and instructions for using the files appear in the link above.
Interesting Related Resources
Screenshots of Mac OS System 1 through Mac OS X 10.3 [Guidebook by Marcin Wichary]
Apple Mac OS X: a history in pictures [Telegraph Media Group Limited]