symfony3中的asset（）函数不转换url

I want to load pictures dynamically from database.

Here's the table containing the image informations:

Controller code

public function listeproductAction()
{
    $em = $this->getDoctrine()->getManager();
    $img = $em->getRepository('AppBundle:image')->findAll();
    return $this->render('lucky/images.html.twig', array(
        'img' => $img ,

    ));

}

twig code

{% for im in img %}
 <img src="{{ asset('/uploads/brochures/' ~  im.getBrochure()) }} ">
{% endfor %}

result

<div class="main-content">
        <img src="/uploads/brochures/0fa58c757752ced32d04b57bb7fa0d65.png ">
    </div>

The problem is that asset function doesn't find the image.