You can do this by changing a registry key. Here are 3 freeware tools for changing Windows 7 logon screen background.
1. Windows 7 Logon Background Changer is free open source software that let you change the wallpaper of the Windows 7 logon screen. (It has been tested successfully with the final version of Windows 7 (RTM) and Windows 7 RC 7100.
2. Tweaks.com Logon Changer lets you choose any jpeg image as log on screen background image. This freeware tool automatically resize and recompress the image to the highest quality possible.
3. 7 Logon Changer is another tool, just select the image (image should be 254Kb) and set it that’s it. To install, right click on the setup exe and choose ‘Run as administrator’, restart or logoff the system to see the changes.