There are two customizations that can be made on the login screen for the Mac OS X. You can change the image used on the screen, and you can add text in the login window. For both of the changes, you need admin privileges. If you have that, here's how to make each of the changes:

For the background image change, do the following:

1 - Open the finder and go to the "Library/Desktop Pictures" directory on the root of your drive (not your user directory).

2 - Rename the file called "Aqua Blue.jpg" to something else. This is the file that is used as the image when logging in.

3 - Take the image you want to use as the new background and name it "Aqua Blue.jpg" in this directory. This image will now be shown as the background screen when logging in.

To add text in the login screen, do the following:

1 - Go to the "Library/Preferences" directory on the root of your drive (not your user directory).

2 - Duplicate the file called "com.apple.loginwindow.plist". Do this will create a new version of the file called "com.apple.loginwindow copy.plist".

3 - Rename the file called "com.apple.loginwindow.plist" to something safe like "com.apple.loginwindow-OLD.plist".

4 - Edit the "com.apple.loginwindow copy.plist" file using the Property List Editor program.

5 - Click on the arrow next to "Root", this will show all of the properties.

6 - Click on Root and then click the "New Child" button in the header of the window.

7 - Where it says "New Item" in the Property List column, change the text to "LoginwindowText"

8 - Leave the "String" value in the Class column as is.

9 - In the value column, enter the text that you want shown on the login screen.

10 - Save the file.

11 - Rename the file to "com.apple.loginwindow.plist".