In the usual case mostly, Windows 7 automatically selects the background for the Login screen which can't be changed unless you hack your way into the system files, but here we have a utility that will make this task easy and simple for you.
Windows 7 Logon Background Changer is a free open source software that let you change the wallpaper of the Windows 7 Login screen. This small program is WPF based decent GPU for the 3D animations to run smoothly.
On the usage part, its pretty straightforward. Run the app, choose a folder that has the images that you want to keep as the background of your Login screen, select the image and click Apply.
It automatically creates a cropped and resized version of the image that you select which is then applied as the background of the Login screen. And the best part is that it does not mess around with the system files so it is completely safe to use.
Give it a try, download it.