In previous versions of Windows, you could use third-party hacking applications to customize the logon background. In Windows 7 there is a manual, no hacking progam necessary, way to do it.
Step 1: Edit the Registry
Open the registry editor (search/run regedit). Go to the directory: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Authentication\
LoginUI\Background . Open the DWORD value OEMBackground. Change its value to 1.
Step 2: Make the Directory
Open C:\Windows\System32\oobe in Windows Explorer. In that location make a folder titled info. In info make a folder titled backgrounds. Most computers will not already have these two folders.
Step 3: Find a Picture
Try to find a picture that is similar resolution to a desktop background or your screen because it will be stretched to fit the screen. Also, the picture must be under 256KB file size to work.
Once you have the picture you want, copy it to the directory C:\Windows\System32\oobe\info\backgrounds . Change its name backgroundDefault.jpg . Logoff or restart your computer to test it.