Discussion settings for new images uploaded to a post should match the comment settings of the post by default, not the wider Settings → Discussion. This is because sometimes when you are adding new images to a gallery you will want to have comments allowed, and sometimes you won't—regardless of the options in Settings → Discussion which is for posts.
Source: http://en.forums.wordpress.com/topic/disabling-comments-in-image-gallery-carousel?replies=20
