The suggested solution will also stop the custom images as well as remove any overlay text and hyperlinks on the course tiles!
To only stop the automatically loaded image backgrounds you should be better off with this:
div.courses-view-course-item a div[style*='data'] {
background-image:none !important;
}
Just tested on the Dashboard in My Moodle 3.5.1. Not sure I want this setting, but I found it an interesting case.