Hello, Kent!
The footer background image is added via CSS. In order to replace it add the following CSS code under Appearance ->Customize ->Additional CSS:
.site-info::before {
background: url("images/footer_skyline.png");
}
Replace images/footer_skyline.png with the direct path to the desired image. You may get the full image path in your Media Library by clicking on the image thumbnail – you will see the path at the right: https://prnt.sc/shgta1.
Regards,
Andre