You can change the logon screen by navigating to this key: HKEYLOCALMACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background. Change the value of OEMBackground to 1. Then go to C:\Windows\System32\oobe and create a folder called info. Inside this folder, create another one called background and place the image in that folder and you will have the image changed.
The welcome screen can be changed with a 3rd party tool but it'll most likely damage Windows installation.