In a previous guide I explained How to Change Ubuntu Logon Screen Background Image (for Ubuntu 11.04 and previous versions). In Ubuntu 11.10 Oneiric Ocelot GDM (Gnome Display Manager) has been replaced with LightDM. Keeping this in view, a third party developer has created a logon screen changer known as Simple LightDM Manager. This application provides a GUI (Graphical user Interface) for changing the Ubuntu 11.10 logon screen background image and logo.
You can install LightDM by entering the following commands in a Terminal window:
sudo apt-add-repository ppa:claudiocn/slm sudo apt-get update sudo apt-get install simple-lightdm-manager
To change Ubuntu 11.10 logon screen background image, select an image or solid color for the logon screen background and optionally a logo. Click Apply Changes and restart your computer for the changes to take effect.
Once your system reboots, the new logon screen image will be automatically applied. Unfortunately there is no option to revert the changes applied by Simple LightDM Manager application and the only way to get back the old logon screen, is to manually select the previous background screen image and logo.
Using Simple LightDM Manager application, you can change your logon screen look periodically by applying exotic imagery. I was able to successfully use HD wallpapers as logon screen background images. I would advise you to use large images for your logon screen background, to avoid a blur look. Other than the PPA given above, you can also download the Deb Package for Simple LightDM Manager from the Launchpad (download) link given below.