the header element places the image as a CSS background-image.
So you would have to write a CSS media query to swap the background-image: url('my-retina-image-url');
This article provides the retina resolution media queries:
Thanks, David. What you wrote and the info in the link was largely undecipherable to me as a newbie. 🙁 I don’t have a clue what or where to put in the CSS for say my home page — just the lines from the first section of the article you linked to? Or the line you wrote above somewhere? ??? Oh well. I thought there might be an easy way to do it. Thanks.