Change Default User Avatar

// Change Default User Icon
add_filter( 'avatar_defaults', 'wpb_new_gravatar' );
function wpb_new_gravatar ($avatar_defaults) {
    $myAvatarID = '78' //CHANGE TO YOUR IMAGE ID
    $myAvatarURL= wp_get_attachment_url($myAvatarID);

    $avatar_defaults[$myAvatarURL] = "Default Gravatar";
    return $avatar_defaults;
}