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

You need to loop through the attachments within your post loop, replace the section of code you posted with this (put this together from some other code I found related to a similar problem, but couldn’t test it):

</BasicDetails>
<?php  $args = array(
            'post_parent'    => $post->ID,
            'post_type'      => 'attachment',
            'numberposts'    => -1, // show all
            'post_status'    => 'any',
            'post_mime_type' => 'image',
            'orderby'        => 'menu_order',
            'order'           => 'ASC'
       );

$images = get_posts($args);
if($images) { ?>
<Pictures>
  <?php foreach($images as $image) { ?>
   <Picture>
    <PictureUrl><?php echo wp_get_attachment_url($image->ID); ?></PictureUrl>
     <Caption><?php echo $image->post_excerpt; ?></Caption>
  </Picture>
  <?php } ?>
</Pictures>
<?php } ?>
<Agent>

EDIT – Updated based on asker edits.