How can I include a photo into my Mobile Pop-Up Banner?

Site URL: https://www.wwo.org

Hi! I have a promotional pop-up on my site that is just a square graphic along with two buttons. It looks fine on desktop, but the mobile pop-up does not show the graphic, just the two buttons. Is there anyway to customize the CSS so that I can have that square graphic appear above the mobile pop-up? Any help would be greatly appreciated!

Try adding to Design > Custom CSS > Replace with your image url

@media screen and (max-width:767px) {
div.sqs-slice-group.group-copy.copy-layer-background {
    background-image: url(https://cdn.pixabay.com/photo/2018/03/11/14/09/eggs-3216877__340.jpg) !important;
    background-size: cover;
    background-repeat: no-repeat;
}
}