Heres a quick how-to guide to teach anyone who doesnt know how, how to customise the login screen in windows 7. before you do anything else, choose the picture you want to use, it must be smaller than 250k, JPG and should be named 'backgrounddefault'.
now, download this (note: you'll need WinRAR or something similar):
once you have downloaded and extracted it, run the program (it installs the correct key into your registry editor).
now, navigate to this directory: C:\Windows\System32\oobe
if you dont already see one, create a folder called 'Info'. If you have a factory-installed OS, this folder will probably already exist, and contain OEM images. Inside the Info folder, create another folder called 'backgrounds'.
now, copy the image you prepared earlier into the backgrounds folder and reboot. Aslong as the Info folder contains the backgrounds folder, and the backgrounds folder contains your JPG image called backgrounddefault, it should now be your login background!