Disregard my last request, I figured it out. For anyone wondering, I replaced the "bg-modal-top-dark" JPG in the filesystem with my new image. The file is located in /var/www/fastuser/data/www/public_html/static/images.
@Tech Support I did find a potential issue you might want to look into fixing. The stock background image for the pop-up is out of alignment and doesn't fill the pop-up window, please see attached screenshot.