My self-created header image 760x130 does show up, super-imposed on the Kubrick 760x200, and the remaining 70 pixels of Kubrick show above my image.
What I did was this: I turned off the Kubrick graphic in admin which used to also show on top of my image. That worked.
Then I changed the height in #headerimg in style.css to 130 but left width intact since that matches the Kubrick's 760.
I gave my image the name kubrickheader.jpg and renamed the 'real' Kubrick to oldkubrickheader.jpg just in case 'something' in the theme looks for that name. I suspected that header-img.php would otherwise rebuild the old scheme, from the way the code looks there.
How can I eliminate the blue 70 pixels of the blue Kubrick showing and also have my image then move up by 70 px now occupied by the 70 blue pixels?.
Many Kudos to a helpful soul.