I ran into the same problem so I searched the sources for "directory listing denied" and here is what I found.
public function index(HTTPRequest $request)
// return 403 error if indexing is not allowed
return new HTTPResponse(_t('ImageCollection.ERROR403', 'Directory listing denied.'), 403);
// render page
In the admin section, the content tab of the page you created will have a checkbox for Allow Index, also labeled "When indexing is allowed, an index page with all images will be displayed if the page URL is loaded directly." Check this box and you will be able to go to the page. Otherwise, I believe you need to add the filename of the image you want to see.