Hi there Nogorth,
this is actually quite simple:
First, place the .bmp image you
would like to see at the log on screen inside the system32 folder: C:\WINDOWS\system32
then, run regedt32 from the start/run menu. Expand KEY_USERS, then expand .DEFAULT. Next, expand Control Panel and select Desktop. Now, in the right hand pane, double click Wallpaper. This will open a new window entitled "Edit String". Now enter the name of the .bmp image you placed in the System32 folder under the value "Value data".
This will set the log on wallpaper for all users.
Hope this helps.

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