It sounds like you want to change the background that is shown on the logon page. In Windows 7, that is fairly easy to do, but requires modifying the registry and adding some files to the Windows folder. There are 2 steps:
1. run regedit.exe and browse to:
Create a DWORD called OEMBackground and set its value to 1
2. Browse to C:\Windows\System32\oobe\info\backgrounds. (You may have to create some of these folders). Create a file there called backgrounddefault.jpg. The file must be less than 256KB and must be a .jpg file.
A more complete description is available here: http://www.makeuseof.com/tag/how-to-change-windows-7-logon-screen/
This video also shows how to do it:http://www.youtube.com/watch?v=e9c_z8nFJIA
You can also search the web for something like "OEM branding windows 7 logon screen" in your native language and might be able to find something that is easier to follow.
Hope that helps