Hi mashupmom.
Thank you for contacting us and for using our theme. We appreciate it.
The original size of that image is 1200x550px, as the notice in Appearance -> Customize -> Header Image states.
You can upload an image that is shorter vertically, like 450px or even shorter and add this to Appearance -> Customize -> Additional CSS:
.frontpage-banner > img {
height: 450px;
width: 100%;
}
Adjust 450px to the height of the image you’re going to use.
Hope this helps but if you have any additional question, feel free to ask.
Kind regards.
Thanks — I tried doing this by uploading an image that was proportional to the original header but smaller, but it seemed to stretch the banner to the width of the page and distort.
Then I tried uploading one that was 400×1200 and it looked great on desktop, but on mobile distorted into a weird square shape?
You’re welcome.
If you’re happy with the appearance on a desktop please apply it and let me know when in place so I could see how does it look on mobile and come up with some CSS that would make it look good there too.
Kind regards.