Windows 7 Logon Background Changer is a free open source software that let you change the wallpaper of the Windows 7 logon screen (also known as "welcome screen", "login screen" or LogonUI).

It works with Windows 7 Home Basic, Home Premium, Professional, Ultimate, Enterprise and Starter, in x86 or x64 (32 or 64 bits). It also works on Windows Server 2008 R2 (but you are not supposed to customize a server).

On a side note, this small program is WPF based, it's a nice technical demo of Windows Presentation Foundation capabilities for those interested in WPF. It requires a decent GPU for the 3D animations to run smoothly. Learn more.