What size should my course image be?
Every course can and should have a course image. That’s the image that’s displayed in the list of courses on the All Courses and My Courses page. It’s also the large image displayed on the individual course page and in the enrollment email when someone is added to a course.
That image should be 1280×720 to avoid awkward stretching or squashing in some browsers and email clients.
Here is a sample course image for reference.