//place this in your functions.php file
<?php 
add_filter('avatar_defaults', 'newgravatar'); function newgravatar ($avatar_defaults) {$myavatar = get_bloginfo('template_directory') . '/images/default_gravatar.png';     $avatar_defaults[$myavatar] = "yournamehere"; return $avatar_defaults;}
?>
 
//add this where you want your gravatar, notice the default url "<pathtourl>" removed
<?php echo get_avatar($comment,$size='48',$default='' ); ?></div>

Gives you the option to select a custom gravatar in the Wordpress discussion setting in admin dashboard

WORDPRESS CUSTOM DEFAULT GRAVATAR

wordpress

PHP