Altering the Login Screen

Now that you have your desktop configured, it is time to do something about that login screen. Every Ubuntu version has a different default login screen. How you change it depends on the version of Ubuntu.

With every Ubuntu version up through Jaunty Jackalope, including Dapper Drake (6.06 LTS) and Hardy Heron (8.04 LTS), you can open up the Login
Window Preferences applet (System ? Administration ? LoginWindow). This lets you select different styles, themes, backgrounds, colors, and even security
options such as an automatic login and the list of users who are permitted to log in.

Beginning with Karmic Koala (9.10), the login screen settings have become significantly feature limited. The only thing you can do is enable or disable the automatic login.Under Karmic, you will need to change the login configuration manually. You can change the background, colors, widget set, and fonts by running the gnome-appearance-properties applet as the Gnome Display Manager (user gdm).

sudo -u gdm dbus-launch gnome-appearance-properties

Alternately, you can change specific properties using gconftool-2. For example, you can get and set the login screen’s background image using:

# Get the current background image filename
sudo -u gdm gconftool-2 -g /desktop/gnome/background/picture_filename
# Set the background image to a new file
sudo -u gdm gconftool-2 -s /desktop/gnome/background/picture_filename \
-t string /path/to/background.png

The other aspect of the login screen that I usually change is the user list. By
default, it lists every user on the system. However, if you want your privacy
(and to not show every name on your system) or have dozens of users on the
system and don’t want a long scroll window, then you will need to disable the
user list.

# Get the current user list setting
sudo -u gdm gconftool-2 -g /apps/gdm/simple-greeter/disable_user_list
# Set the value to true for disabling the list
sudo -u gdm gconftool-2 -s /apps/gdm/simple-greeter/disable_user_list \
-t bool true

You can alter other options using the graphical gconf-editor. I suggest
looking under the configuration path /apps/gdm/simple-greeter. This is
where you can find options to set the greeter text message, change language
settings, and even disable the login screen’s restart button.

sudo -u gdm dbus-launch gconf-editor

After you make these changes, log out and view your handiwork.

Source of Information : Wiley Ubuntu Powerful Hacks And Customizations