You can change your Windows log-on screen as follow:
1. Go to start < Run < type Regedit and press enter.
2. Browse to : HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background
3. Open the ‘OEMBackground DWORD’ key
4. Set the key value to 1.
5. Select a background image for your login Screen (256KB at most).
6. Rename the image to backgroundDefault.jpg
7. Copy your image to the C:\Windows\system32\oobe\info\backgrounds folder
8. Reset the computer.