By default the first option under the Fit settings is selected. This setting keeps the image at its own aspect ratio but only allows it to expand based on its own maximum size. In some display uses, the image could actually not fill the whole space available in the section.
We want the image to be able to fit the Section but not distort its aspect ratio.
Select the third Fit setting to do this.
Click on The background image maintains its aspect ratio and fits its smallest dimension to the surface, causing the larger dimension to overflow