51 is the node id of the image folder & 5 for images (check the id node of your install and change the array to the class you want to fetch. You also have to give access to "Anonymous Users" to the media folder at "Roles and policies".
The site center there is banner for the site withe latest 5 articles, random showing, and also there is banners for advertising, also there is mix images use in the site.
I had create folder called site banners under images ... I couldn`t fetch this folder, until I moved to media/banners !
that was the problem .. solved by moving it folder banner ^^