Windows 7 have own Logon UI customizable option. The new operating system support loading images as background for login screen without using third-party software.
Be careful, you have to change something in the Windows Registry, proceed with your own responsability.

Open the registry (start -> run -> regedit or WindowsKey -> regedit) 
find the key HKLM\Software\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background
if not present [...]