2. Move your wallpaper ,run following command in terminal
(to open terminal go to Applications->Accessories->terminal)
sudo mv ~/your-wallpaper-name.jpg /usr/share/backgrounds
3. Run the following command to activate the Appearance window upon login
sudo cp /usr/share/applications/gnome-appearance-properties.desktop /usr/share/gdm/autostart/LoginWindow
4. Now close the terminal and logoff .(If you can’t find your favorite wallpaper,
click Add. You should be able to find your wallpaper in the /usr/share
/backgrounds directory).5. Your login background will instantly change to the wallpaper you have
selected. Now login to your account.
6. Open terminal. Type the following command to deactivate the Appearance
window upon login.
sudo unlink /usr/share/gdm/autostart/LoginWindow/gnome-appearance-
You are done ....Enjoy :-)