Within Windows reports that the latest build of Windows 7 support logon UI background customization
>> Windows 7 to officially support logon UI background customization
As you probably know, Windows 7 build 7057, a build recently leaked to the public, sports a new (yet ugly) login UI background. Delivering on the “more customizable” promise, Windows 7 now supports the ability to load images into the background of the login screen without the use of third-party software or manual hacks.
Although this functionality was designed for OEMs in mind, it is pretty easy to turn on and off using regedit and some images lying around your hard drive.
>> Windows 7 to officially support logon UI background customization