One of the lesser-known improvements (to me, anyway) that WordPress.com implemented at the end of November 2012 was the ability to manage Discussion Settings on images, in addition to Posts and Pages. As a result, you now can allow or disallow comments and pingbacks on images almost as easily as "Likes" and Shares".
Comments are enabled by default on your site globally (under Settings>Discussion Settings>"Default article settings"), meaning that commenting is available on Posts, Pages…
Jen, from Random Acts Of Photography, on tuning comments
Glad you bring this up because I have noticed it, like it very much – so far, but few are using it.