This is a combined result from what i searched in the net,to change logon screen manually you got to enter Regedit in the textfield on the window7 start menu, go into the path Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background ,If an OEMBackground DWORD entry already exists, set the value of OEMBackground to 1. If not, Right-Click on the "Background" folder and select new DWORD. Set the name to OEMBackground and its value to 1 .
now go to c:\windows\system32\oobe
if any
folder named Info then just click in .if not create one folder name Info .
inside the folder should have another folder named backgrounds,if no just create yourself ,and now you almost done .
just one more step ,pick a right picture in JPG format and size should be less than 256KB then move into the backgrounds folder and rename the wallpaper as backgroundDefault.Then u are done.If the C:\Windows\System32\oobe\Info\Backgrounds folder already exist any JPG wallpaper ,just delete it and replace in with your wallpaper.
backgroundDefault.jpg should be in these ratio (width-height)
* background768×1280.jpg (0.6)
* background900×1440.jpg (0.625)
* background960×1280.jpg (0.75)
* background1024×1280.jpg (0.8)
* background1280×1024.jpg (1.25)
* background1024×768.jpg (1.33-)
* background1280×960.jpg (1.33-)
* background1600×1200.jpg (1.33-)
* background1440×900.jpg (1.6)
* background1920×1200.jpg (1.6)
* background1280×768.jpg (1.66-)
* background1360×768.jpg (1.770833-)
