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

C:\windows\system32\obe\Info

Now create  a folder called backgrounds.

C:\windows\system32\obe\Info\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.

Download Registry Scripthref>

Hope this helps