The default wordpress themes are came with avatar support. In case you have use some other free or premium wordpress themes with no avatar support, then you can adding the following simple and short coding to your comment.

<?php echo get_avatar( get_the_author_email(), '80'); ?>

get_the_author_email outputs the post author’s email and the “80” is the size of the avatar image in pixels, you can change the size of the avatar according to your needs. Is it looks simple? @@