The footer background image options can be found in Customize > Background Images.
The footer area option is for the copyright.
If you need to set background images for each individual footer widget, you’d have to use some CSS like this:
.site-footer .footer-widgets-container .inner-padding {
padding: 40px;
background-image: url(URL TO YOUR IMAGE);
margin: 10px;
}
.site-footer .footer-widgets-container .inside-footer-widgets {
margin: 0;
}