Good news: In former times you had to disable System Restore, exchange some dlls n’stuff. Today Windows 7 allows you to change the background of the Logon Screen pretty easy:

It’s only the the following 2 steps:
1) set the value of the registry key from 0 to 1: HKLM\Software\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background\OEMBackground (dword)
2) copy the desired new background image named “backgroundDefault.jpg” into C:\Windows\System32\oobe\info\backgrounds

Et voilà: the background has changed without any restart!

Note: create the registry key or folder, if it does not exist!
Note2: imagesize may not exceed 256k !

Source: (Grazie mille!)