I just upgraded to 14.04, and it's amazing. Obviously, I went to check out the new backgrounds and one caught my eye. It's an abstract blue design and is marked as one that changes throughout the day and its title is "Default Wallpaper". I really would like to find this wallpaper's file, but I can't.
If you are wondering, it looks like this:
I've checked in
/usr/share/wallpapers but neither has them, just the picture wallpaper. Is there a special place that the ones that change are stored, or are they simply inaccessible?