I am running Ubuntu 12.04 with gnome-shell. I have two admin user accounts. Now, I used to know that in Precise we can set different different login wallpaper to different different user, so I tried with:

sudo dbus-send --system --print-reply --dest=org.freedesktop.Accounts /org/freedesktop/Accounts/User1001 org.freedesktop.Accounts.User.SetBackgroundFile string:/usr/share/backgrounds/Green-wallpaper-27.jpg

but to no avail.

Then I tried

sudo xhost +SI:localuser:lightdm
sudo su lightdm -s /bin/bash
gsettings set com.canonical.unity-greeter background '/path/to/the/wallpaper.png'

It does change my login wallpaper but for all users.

Then I tried to locate /etc/lightdm/unity-greeter.conf but I found that it is not there. So where is it and how can I set different login wallpapers for different users?