Replace the default "snowman" avatar with a custom image. Code lifted from this post -> http://www.bourncreative.com/how-to-create-a-custom-default-avatar-for-wordpress

custom-gravatar.php

<?php //Remove opening tag

function add_custom_gravatar( $avatar_defaults ) {

     $myavatar = get_stylesheet_directory_uri() . '/images/custom-gravatar.jpg';
     $avatar_defaults[$myavatar] = "Custom Gravatar";
     return $avatar_defaults;

}
add_filter( 'avatar_defaults', 'add_custom_gravatar' );