the image of the sub category or the image of the parent category
this is correct for the parent term
$image = get_field('category_image', $current_cat );
this would be used for the child term
$image = get_field('category_image', $child );
however, this may not be returning what you expect it to return, and what it is returning would depend on where you are adding the shortcode.
$current_cat = get_queried_object();