Thank you catymag!
I couldnt make it work when using gmaptool with the GUI, but I was succesful when using the command line version.
Now, if I were to use the patched mapsource 6.16.1 - will the procedure with the img files be the same on a win7 x64 machine? I tried it but couldn't figure out how to make it work. Since the maps are stored in another place I guess the install.bat needs some tweaking...right?