A while back I wrote a post on how you can change the login screen for Windows 7, or at least the wallpaper used on the login screen. That article is still valid, but it requires you to do some registry edits which not everyone is comfortable with. I have a solution for you!
Introducing the Windows 7 Logon Background Changer. From the page:
Windows 7 Logon Background Changer is a free open source software that lets you change the wallpaper of the Windows 7 login screen (also known as "welcome screen" or "login screen"). It does not change any system file, and the program itself does not requires admin rights to run (it will just ask you to run as admin a very simple cmd file that creates the required folder and registry key with the appropriate rights) It creates a few JPEG files based on the image you want to put as wallpaper for the Windows 7 login screen, applies the appropriate cropping and sizing and saves them using the best compression quality possible.
Here is a screen shot: