Yeah! thanks harpo. I have the Unit_main.tga, thanks to your exellent tool, I have worked out the problem now. The problem was that Diplomacy has no MainViewIcon-Ship file in it's Window folder, not even in the install folder - ProgramFiles>StardockGames>SoaSE. So there was no brushes refering to the Icons that I had put into the Unit_Main.tga.
So I just copied the the MainViewIcon-Ship file out of Entrenchment, pasted it into the Window folder of my mod, added the location of the Icon's that I had put in the Unit_Main.tga and it works fine.
It is puzzling, how the game knows to put the Icons for the envoy's in, with no MainViewIcon-Ship file in Diplomacy. I guess it must be hardcoded in.