How do you get all the urls of images attached to a post?

I've searched high and low and can't seem to get it.

I'm trying to output an XML feed with all the images attached to a custom post type:

  <Pictures>
   <Picture>
    <PictureUrl><?php echo wp_get_attachment_url( get_post_thumbnail_id($post->ID)); ?></PictureUrl>
     <Caption></Caption>
  </Picture>
  <Picture>
   <PictureUrl></PictureUrl>
    <Caption></Caption>
  </Picture>
 </Pictures>

I'm using wp_get_attachment_url but it's only returning one image:

        <?php echo wp_get_attachment_url( get_post_thumbnail_id($post->ID)); ?>

The <Picture> is a repeating element so it should start an new tree when there's another image attached.

Any help would be Amazing!