Changing the Windows log-on screen used to involve some complicated and potentially dangerous hacks, but not any more with OS like Windows 7 or Windows 8.
First, browse to HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion
\Authentication\LogonUI\Background in REGEDIT, double-click the DWORD key called OEMBackground (not there? Create it) and set its value to 1.
Now find a background image you’d like to use. Make sure it’s less than 256KB in size, and matches the aspect ratio of your screen as it’ll be stretched to fit.
Next, copy that image into the %windir%\system32\oobe\info\backgrounds folder (create the info\backgrounds folders if they don’t exist). Rename the image to backgroundDefault.jpg, reboot, and you should now have a custom log-on image.
Alternatively, use a free tweaking tool to handle everything for you. A software called Logon Changer displays a preview so you can see how the log-on screen will look without rebooting, while a software called Logon Screen Rotator accepts multiple images and will display a different one every time you log on.