I've stumbled upon your question here and although I'm not sure if I can be of any help at all (I'm new myself here and learning), may I share with you the following:
I've recently changed to Chateau and what I decided to do was to use a Sticky Post where I inserted an image. This is my "Welcome" message, so it will stay at the top of front page. You can choose the size of that image. I chose small/medium as I don't want to overflow the Welcome message itself.
I'm not sure if that's what you want: a static image on your front page which is NOT the header image.