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.

1. Open the registry (start -> run -> regedit or WindowsKey -> regedit) 
2. find the key HKLM\Software\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background
3. if not present create a new DWORD called OEMBackground with value 1 (0 for disable)
4. Registry is done, now we must place the backround in right place. The default folder is %windir%\system32\oobe\info\backgrounds
5. if the directory backgrounds is not present, make new one
6. put inside the new backrounds, but the are some pre-requisites. The file must less than 256KB. The name of file must be backgroundDefault.jpg. Is good idea make backround at same resolution of your screen, otherwise image will strech-to-fit

happy tweaking