Here is an impatient guide for changing the background image on your login screen on Windows 7.

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Note: The image file you use should be a .jpg file and it should be less than 256 KB.

1. Press WIN_KEY + R
2. Type regedit, then hit ENTER
3. Set or change the DWORD value HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\ Authentication\LogonUI\Background\OEMBackground to 1
4. Rename your picture file to backgroundDefault.jpg
5. Copy your picture to %windir%\system32\oobe\info\backgrounds directory.  If info\backgrounds doesn't exist you may have to create those directories.  Tip: Press WIN_KEY + R, then enter the above path.  This should open explorer for you.
6. Log off or reboot your machine to see your new authentication/logon background.

This information is curt and used for personal reference.  Use at your own risk.

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If this guide is confusing, use a search engine to find a more detailed explanation.  Cheers!