By default Ubuntu 12.04 uses LightDM as the login manager which looks like the following screenshot. it was very simple to change the login screen background. Now this tutorial explains you how you can easily customize your login screen.
WordPress plugin repository has a new login logo plugin that lets you use a custom login logo without any need to copy-paste logo path or manually upload logo image via FTP. You can easily select a custom login logo with a custom height directly from your admin area dashboard.