Thanks for the reshacker idea. Works like a charm! The bmp now is completely black and can be embedded nicely. If I put a little bit more time in it, it's even possible to give it the stripes of the skin and make them match perfectly. Only problem is as you say, since there is no way to resize it, it doesn't really fit the iDrive design.
I don't have a lot of time now. Maybe later when I have some time I will try to make another "play mode" screen with the analyzer embedded. But to be honest, I think I will not get it in there without messing up the design. For other skins I think this way it could be a nice spectrum analyzer. Gonna give it a try on my friends TR-skin (again, when I've got some time )