I used to have a custom background setup on my Ubuntu 9.10 system. My login screen background was reset to the default purple background after upgrading to Ubuntu 10.04. If you want to setup a custom login screen background there are (at least) two methods.
Install Ubuntu Tweak. Ubuntu Tweak will let you tweak many things including the login screen background.
To install Ubuntu Tweak add the third party repository to your list of repositories.
Go to System-Administration-Software Sources, Third-Party Software tab, Add:
deb http://archive.getdeb.net/ubuntu lucid-getdeb apps
Add the repository GPG key, open a terminal window and type:
$ wget -q -O- http://archive.getdeb.net/getdeb-archive.key | sudo apt-key add -
Install ubuntu-tweak from the command line.
$ sudo apt-get update
$ sudo apt-get install ubuntu-tweak
Copy your favorite background .jpg file to /usr/share/backgrounds
$ sudo cp my_cool_bg.jpg /usr/share/backgrounds
$ sudo cp /usr/share/applications/gnome-appearance-properties.desktop /usr/share/gdm/autostart/LoginWindow
You should see an “Appearance Preferences” window, select the Add button and select the .jpg file that you want for your login screen background. Close the window.
Login again and remove the gnome-appearance-properties.desktop file that you copied in the previous step. This will prevent you from getting the appearance-properties windows at every gdm login.
$ sudo rm /usr/share/gdm/autostart/LoginWindow/gnome-appearance-properties.desktop
Which method should you use? If you really enjoy customizing your GUI and changing its behavior, then use Method 1 and install Ubuntu Tweak and enjoy having the control over many settings within easy reach.