PODS CMS - For Each images not working

       <?php
foreach ($listing->get_field('photos') as $photo) {
  $thumbnail = wp_get_attachment_image_src( $photo[ID], 'thumbnail' );
  $fullsize = wp_get_attachment_image_src( $photo[ID], 'full');
  echo "<li><img src='$thumbnail[0]' alt='$name' width='75' onclick='document.swap.src=\"$fullsize[0]\";' /></li>";
}
?>