Facebook post message for upload with large image?

I tried to post messages automatically on my facebook page. It works great, but I want to use a larger image for general publication. http://cdn.tips4php.net/wp-content/uploads/2010/12/final_post.jpg

I used the following code to post it:

 try {
 $page_info = $facebook->api("/$pageId?fields=access_token");

 $attachment = array(
 'access_token' => $MYACCESTOKEN,
 'message' => $msg,
 'name' => $title,
 'link' => $uri,
 'description' => $desc,
 'picture'=>$pic,
 'actions' => json_encode(array('name' => $action_name,'link' => $action_link))
 );

$status = $facebook->api("/$pageId/feed", "post", $attachment);

 } catch (FacebookApiException $e) {
 error_log($e);
 $user = null;
 }

How can I use a large picture for this post? I saw some general posts from other facebook pages and they have a big photo. Waiting for your reply.