NextGEN Gallery Comments by Roberto Cantarano allows all users toleave comments to your galleries.
Comments are also displayed in admin in Comments section and in manage gallery page. Template : You can add custom comment form template for your theme. (read F.A.Q.)
The plugin requires at least version 1.8.3 of NextGEN Gallery.
Download: http://wordpress.org/extend/plugins/nextgen-gallery-comments/