Ubuntu Wallpapers

You can actually see where the current background is stored, by running (in a terminal window) the command:

gsettings get org.gnome.desktop.background picture-uri

I get for example:

'file:///usr/share/backgrounds/Mono_Lake_by_Angela_Henderson.jpg'

That means my wallpaper is stored in /usr/share/backgrounds. In 14.10, that is the default location for wallpapers.

Default Ubuntu wallpapers are stored in /usr/share/backgrounds/. For each Ubuntu release, there is a new package created for the wallpapers of that release. For 14.10, the package name is ubuntu-wallpapers-utopic, you can get to see the list of wallpapers by issuing the command

dpkg -L ubuntu-wallpapers-utopic | grep .jpg$

You can also find the wallpapers for the 14.10 release in http://packages.ubuntu.com/utopic/all/ubuntu-wallpapers-utopic/filelist (files that end with .jpg).