Windows 7 makes it easy Changing the Windows log-on screen.

Go to start, and click on Run,type Regedit & press enter there.

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 (Use of Image Resize software like VSO Resizer can help)

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.