Sure you can do this via CSS. However, doing so will result in a pixelated image. If you’re anal like me, change the comments.php file (using your child theme, of course) and look for (i.e. do a search for any instance of wp_list_comments :D) a line similar to:
wp_list_comments('avatar_size=48&type=comment');
The avatar_size parameter can be between 0px to 80px.