How To Change Windows 7 Logon Screen

1. HKLMSoftwareMicrosoftWindowsCurrentVersionAuthenticationLogonUIBackground

 - double click on key named OEMBackground and change its value to 1
(If you cannot find the path or the key go ahead and create it, remember OEMBackground is a Dword)

You just enabled the OEM Background functionality. Now browse to the folder

2. %windir%system32oobeinfobackgrounds

- again the folder path may not exist already, so you can go ahead and create required folders (info and backgrounds are generally not present)

3. now put in any JPG file with size less than 256 kb in this folder and rename it to backgroundDefault.jpg

http://www.makeuseof.com/tag/how-to-change-windows-7-logon-screen/