Step 1: Open the Windows Start menu and enter Regedit into the search bar.
Step 2: Right click on HKEY_LOCAL_MACHINE and select Find.
Step 3: Search for OEMBackground, which should be in ComputerHKEY_LOCAL_MACHINE/SOFTWARE/Microsoft/Windows/CurrentVersion/Authentication/LogonUI/Background. If the key does not exist, add a new DWORD value with the name OEMBackground.
Step 4: Double-click on the entry OEMBackground, and change the value from 0 to 1.
Step 5: Open Windows Explorer and go to the following path:%windir%\system32\oobe
Step 6: Create a new folder named info and open it.
Step 7: Create a new folder within info named backgrounds.
Step 8: Rename your desired wallpaper as backgroundDefault.jpg and place it inside the folder backgrounds (images must be less than 245KB in size).
With the help of this trick you can easily change you windows 7 Log On Screen.
Posted by Shubham ;)