I’ve continued to test this. For the retina uploader, it works great. Perfect. No problem. Using the other uploader, I am forced to crop the image to 360 × 70. See http://www.reasonsforgod.org/wp-content/uploads/2015/11/Screen-Shot-2015-11-27-at-11.38.30-PM.png for the window that I see.
I’m likely doing something wrong; I welcome your excellent guidance on how to upload a full-width, normal size header.