Windows 7 Logon Background Changer is a free open source (source code available soon) software that let you change the wallpaper of the Windows 7 login screen. It has been tested successfully with Windows 7 RC 7100, but it should also work with future releases.
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. It runs fine with my Radeon HD 3200 on my laptop computer, but it should also work with lower end graphic cards, albeit slowly.
What does it do?
* It does NOT change any system file, and the program itself does not requires admin rights to run (it will just ask you to run as admin a very simple cmd file that creates the required folder and registry key with the appropriate rights)
* It creates a few JPEG files based on the image you want to put as wallpaper for the Windows 7 login screen, applies the appropriate cropping and sizing and saves them using the best compression quality possible.