photo_comment.php

<?php 

include "facebook/facebook.php";

$facebook = new Facebook( array( 'appId' => APP_ID, 'secret' => APP_SECRET ) );

// enable upload support
$facebook->setFileUploadSupport( true );

// set access token for user / page here (not needed if you intend to use the login flow beforehand)
$facebook->setAccessToken( ACCESS_TOKEN );

// object you want to comment on (can be status update, photo, link etc)
$object_id = '60506094_10100351413616696';

$comment_with_url = $facebook->api( $object_id . "/comments", "POST", array(
    'attachment_url' => 'http://healthhub.co/wp-content/uploads/2014/02/Group-Slider.jpg',
    'message' => "Join the team!"
  ) );

// will return id of comment on success
print_r( $comment_with_url );