Since Ubuntu 11.10 you can see the change in the login screen. If you want to give it more fancy look by using your custom image here is a quick tutorial for this process. Don’t worry you don’t have to mess with several nasty commands. This process is siimple and works with just edit of one files or you can use application which rolled down your new background in a minute. Before getting started I have assume that you are using Ubuntu 11.10 with LightDM as your default display management.
Open the terminal(CTRl+ALT+T) and type the command command.
gksu gedit /etc/lightdm/unity-greeter.conf
The pop up will display . See the image path. You have to modify the image path with you desired image link. You can do that in current directory but note that you can’t put image file in background directory . If you try to put some image you will get permission denied message.
The following command will make changes the ownership of the backgrounds folder to your user, the -R option makes the command recursive so all permissions of folders and files inside background are changed as well.Now you can paste any image file under it. To know more about chmod command see it here
sudo chmod 777 -R /usr/share/backgrounds/
Put your desired image and change the image path and Enjoy the new background at your login screen
Another way: With SimpleDM Manager
If you don’t like to edit the code and type nasty commands you can use SimpleDM Manager. To install this application open you terminal and enter the following command
sudo apt-add-repository ppa:claudiocn/slm sudo apt-get update sudo apt-get install simple-lightdm-manager
This application is simple and easy to use. You dont’ have to messed up with command for using this application. User can select either logo or background color and Apply changes to take you new background into effect However if you want to rollback to your default you can also gain it back by uploading the same image file.