gistfile1.php

<?php if (have_posts()): while (have_posts()): the_post(); ?>
  <?php preg_match('/<img\ssrc="(http:\/\/farm\d+\.static\.flickr\.com\/.*\.jpg)/', $post->post_content, $matches); ?>
  <?php if($matches[1]): ?>
    <?php $thumbnail_image_url = preg_replace('/_.\.jpg$/', '_s.jpg', $matches[1]); ?>
    <img src="<?php echo $thumbnail_image_url; ?>" width="75" height="75" />
  <?php endif ?>
<?php endwhile; endif ?>