I\'m able to get a picture-url from a specific facebook-post through the facebook graph-api (ie https://developers.facebook.com/tools/explorer?method=GET&path=19611830383339
You can simply get the object_id if the post type is a photo directly on the first request to graph api by passing arguments like this
https://graph.facebook.com/$id/posts?fields=id,message,created_time,type,object_id&access_token=$accessToken