The ALX400R has a Krüger+ display by default, so it can use custom BMP displays out of the box. The C400R is the one that can't by default, as it has a busfanat vollmatrix display that doesn't support BMP images.
I did write a tutorial for the EN/GN92 right after the release of OMSI 2, but it was only ever linked from Aussiex and is showing its age rather now... I'll see about tidying it up and linking it from FF.