1. As admin/root, create the directory /etc/guest-session/skel – sudo mkdir /etc/guest-session/skel
  2. While logged on under an admin account, start a guest session.

  3. Make any changes to the guest session that you want using whatever tools/utilities you wish – wallpaper, default home-page etc.

  4. Do not log out, instead switch user back to admin account and open a terminal.

  5. Use the who command to find out the current logged in users. One will be root and the other guest-XXXXX.

  6. Copy the files in the temporary guest home directory to /etc/guest-session/skel using the command: sudo cp -rT /tmp/guest-XXXXX /etc/guest-session/skel

  7. Switch user again to the guest account, log out. Re-login to a new guest-session to check.

  8. Works in Ubuntu 14.04, 15.10 and 16.04. Untested in the rest.