You can use the Blogs Comments Viewer webpart. In your transformation for the blog comments, you need to make sure you are pull the CommentUserName field for the image.
<%# GetUserAvatarImage(50, HTMLEncode(GetNotEmpty("CommentUserName"))) %>
You can find some more Avatar functions here:
https://docs.kentico.com/display/K82/Displaying+avatars+in+transformations