the following css should hide the button to upload/change a course image:
.CourseImageSelector button {visibility:hidden}
How with that being said, if you were to put that in your theme css, it would be hidden for all users, even admins. To do this based more on role you could do this with javascript (either hide the button for all non-admins, or hide for everyone in css, then unhide for admins with javascript). The latter option would look something like (writing this without testing, which is always a bit dangerous):
if (ENV.current_user_roles.includes('admin')) {
const CourseImageButton=document.querySelectorAll('.CourseImageSelector button p').forEach(element => {
element.style.visibility='visible';
});
}
Best practice would be to put this javascript inside an if statement checking if the url is a course settings page.
Hope this helps!