@richard, I have a partial answer but the same question.
You can add an image file to the "Edit course settings --> Course summary files" you can add an image of your choice and this will replace the header you see above. The problem is, it ALSO replaced the image on the Course Tile (course cover?) that by default is an entirely different image (as is seen above, and see below for the issue).
In addition, the resolution the default header is 1500x700 (@168 ppi), but you need to ensure that the image is centred just so in order that it appears nicely with the the DIV block. See below where this hasn't worked.
FWIW, I have added the original Header image below. You might be able to replace this directly, but I think the better way would be to be given some guidance on the GUI, plus the ability to choose a course header image AND a course cover image