Windows 7 Logon Background Changer is a free open source software developed by Julien Manici.
This is almost similar with the desktop or wallpaper changer- only that this software let you change your desired background image in the logon or welcome screen. This software is tested free from spyware, trojans, or other malicious programs. This Logon Background Changer has been tested to work successfully on both Windows 7 RTM and Windows 7 RC.
According to the Julien-Manici.com, Windows 7 Logon Background Changer 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. This software also creates a few JPEG files based on the image you want to put as wallpaper for the Windows 7 login screen, then applies the appropriate cropping and sizing, and saves them using the best compression quality possible.
You can read the thorough and step-by-step instruction on how to install the Windows 7 Logon Background changer on this post.