To set the login background for windows 7 … that is the background image displayed on the login screen …

First – put your image in:

C:\Windows\System32\oobe\info\backgrounds\backgroundDefault.jpg

(you will likely have to create this directory structure …)

Then create/edit a DWORD – 32bit registry key “OEMBackground” and set it to 1 in:

[hklm\SOFTWARE\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background\

reboot …

For more info see:
http://www.intowindows.com/how-to-change-windows-7-logon-screen-easily-without-using-hacks-tools/