Where are the default (system) wallpaper folders located on OS X?
Its on:

/Library/Desktop Pictures/

Notice though this is the system library, not the user library, so its right on the root of the drive and not inside your user folder.

Its hidden by default. Go to terminal and type open /Library to open it.

Hope it helps!



Edit: On Finder, you can hit CMDShiftG and paste the path to go to the folder. (thanks Sjoerd)