Here are two great articles on changing the login screen for Windows 7
A tool to help make the process easier:
http://tweaks.com/software/tweakslogon/
And article that describes the process
http://www.intowindows.com/how-to-change-windows-7-logon-screen-easily-without-using-hacks-tools/
http://www.intowindows.com/download-windows-7-logon-screen-changer-now/