I am currently building a Steam Machine with Windows 8.1, so I try to hide as much as possible Windows

Currently only one element is a problem for me, the user picture on the logon Screen.
I know it was possible to remove Windows 7 via editing the file authui.dll, but in Windows 8.1 this method is no longer functional

I'm looking for a way to remove it or just hide it ? (I already try to put a black image, but there's still the edges that are still visible.)

thank you. (...and sorry for possible spelling mistakes)