There isn't currently a setting to do this, but can be achieved with some custom CSS.
First, upload an image to a banner module. We would suggest uploading at 4,000 px; ideally square, so that it doesn't get cropped heavily when in portrait mode.
Copy the image URL from the uploaded image by right clicking on it.
background-image: url(**insert URL here**);
background-position: top center;
fixed will mean the background doesn't scroll with the rest of the page
top fixes the image to the top of the page and crops from the bottom