Get a thumbnail from pages with The Loop

Hi,
I want to use The Loop to get the first thumbnail from some pages. Right now I am using a custom field ‘picture’ with the thumbnail url.

Is there a way to get the first picture associated with a page? It should also be the ‘thumbnail’ version.

https://moatfarm.org/movingartseast.org.uk

Right now I have this:

<?php $args = array(
        'order'=>ASC,
      	'post_type' => 'page', // Only get pages
        'post_parent' => 2 // Get pages that are children of page 2.
        ); query_posts($args); ?>   

<?php while (have_posts()) : the_post(); ?>
<img src="<?php
   // Get url from custom field 'picture'
   $key="picture";
   echo get_post_meta($post->ID, $key, true);
?>" />

<?php endwhile;?>

Any help sincerely appreciated. I have done as much as I know how and now I need some help.

Charley