In my case, this happened when I changed the value of-
Site Administration -> Appearance -> Courses -> Course summary files extensions.
It should be set to its default value, which is, ".jpg,.gif,.png".
- Go to a particular course.
- Go to Course administration -> Edit Settings.
- Here, under Description -> Course summary files, you can attach image.
- This image is displayed on site home page under 'Courses' section for this particular course.
Hope this helps!