This tweak will only work if you have the "Use welcome screen" option activated in Control Panel>Users Accounts>Logon and Logoff Options.
1. Make a backup copy of C:\windows\system32\logonui.exe
2. Overwrite the file with your new login screen. (be sure to use the same file name as the old logonui.exe)
3. If you ever want to revert back to the old login screen, just overwrite C:\windows\system32\logonui.exe with your backup.
Enjoy!
Some edits and supplements
Windows System File Protection will not allow to override the file unless you follow these steps:
1. Rename the logonui.exe file in either C:\Windows\system32\dllcache or in C:\Windows\ServicePackFiles\i386 (depends on if and how you installed the servicepack).
2. Copy your custom logonui.exe to this folder.
3. Boot into safe mode and do exactly the same (rename the original file and copy in your new one) in C:\Windows\system32. System File Protection will pop up two windows. Answer them with "cancel" and "yes".
Note: It is always better to rename the original file and not override it.
Alternatively you can use this method to change the Welcome screen :
1. Copy your custom logon file under another name, let's say logonuiA.exe, to C:\Windows\system32.
2. Open regedit (Always backup before changing anything in the registry) to
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon
and change the value for UIHost to logonuiA.exe.
