Just second you with the same issue and couldn't find any solution.
Looking at a tracker item, it looks like there is some re-working on this but no mention of replacing images. https://tracker.moodle.org/browse/MDL-62683
Do you want to set one default image for all courses or have a page where you can put image a to course 1, image b to course 2 etc.? In case you want to have 1 default image, you could use the course templates plugin from the moodle db and in the template course at your default image. Then all courses which you create in the future based on the template course would have that image.
This will be possible in 3.7 - see https://moodle.org/mod/forum/discuss.php?d=385813#p1556512