Thanks maverick, your solution works
<?php $args = array(
'post_parent' => $post->ID,
'post_type' => 'attachment',
'numberposts' => 15, // -1, show all
'post_status' => 'any',
'post_mime_type' => 'image',
'orderby' => 'menu_order',
'order' => 'ASC'
);
$images = get_posts($args);
if($images) { ?>
<images>
<?php foreach($images as $image) { ?>
<image><![CDATA[<?php echo wp_get_attachment_image_src( $image->ID, 'listing_gallery_2x')[0]; ?>]]></image>
<?php } ?>
</images>
<?php } ?>