You can change the login background in windows 7 without any hacks or software.
Go to : C:\windows\system32\oobe
Create a new folder called Info
Now create a folder called backgrounds.
Put the picture that you want to have as a background for login screen with name : backgroundDefault.jpg
Note : The picture name should be backgroundDefault.jpg and the size should be less than 256k and format should be jpg.
Now you need to run this registry script to have this work, and then reboot the system.
Hope this helps