Customize the Windows 7 log-on screen
Windows 7 makes it easy Changing the Windows log-on screen.
Go to start, and click on Run,type Regedit & press enter there. This will open Registry.
Navigate to : HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Authentication\ LogonUI\Background
Double-click the OEMBackground DWORD key
Set value of the key to 1.
Select a background image for Logon Screen with size less than 256KB in size
Copy that image into the C:\Windows\system32\oobe\info\backgrounds folder
Rename the image to backgroundDefault.jpg
Reboot, and now your logon Image would have changed.