Here's an interesting Windows 7 tweak that allows you to customize your winodws7 logon screen.
You can do this with Tweak.com LOGON SCREEN CHANGER. It offers a very easy-to-use interface from which you can easily set a new logon screen for your Windows 7. What makes it different from the other logon changers is that it is portable and shows you the preview of your current and new logon screen.
Just run the tool and you will find the following screen.
Simply hit the Change Logon Screen button, browse and select the new image and it will become your new Logon screen.
Make sure that the background you select is smaller than 256KB, otherwise it will be rejected because of a size limitation set by Microsoft.
If you want to remove your customized background just click Revert to Default Logon Screen and the registry changes will be removed.