You should see a DWORD value named OEMBackground.

Now navigate to the following key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Authenticatation\LogonUI\Background. To start off you will need to launch the Registry Editor by typing regedit into the search box in the Start menu and pressing Enter. You need to change a registry value and put an image file in the correct location. This setting was originally intended for OEMs (original equipment manufacturers) to customize their systems. The setting to do this is hidden though, so you need to dig around a while. This can be accomplished without the use of any third-party software. One of the cool things you can do in Windows 7 is to change the welcome screen, which appears when you start up your computer. With a simple tweak you can set your logon image to whatever you like.