Till Windows Vista & Xp there was no support for changing Login Screen background and you have to use third party apps like logon studio for changing login screen background. However in Windows 7 Microsoft has done good job by allowing users to change Windows 7 Logon Screen without need of third party apps.
Here is a cool utility Windows 7 Logon Screen Rotator which lets you to rotate Windows 7 Logon background screen randomly. You can select up to 10 images and Logon Screen Rotator will automatically rotate your Windows 7 logon screen.
