The code to call the normal Thesis Thumb is at http://snipplr.com/view/38971/thesis-thumbnail/\r\n\r\nTo test whether there is a thumbnail, and if not use another one, use this code in custom_functions.php.\r\n\r\nYou can change the class alignleft to alignright, or aligncenter and you can add the class frame to give the thumbnail a border.

function custom_thesis_thumb() {
  global $post;
  $post_image = thesis_post_image_info('thumb');
  if (!$post_image['url'])
    echo '<a href="' . get_permalink($post->ID) . '"><img src="YOUR DEFAULT THUMB URL" class="thumb alignleft" /></a>';
}
add_action('thesis_hook_before_teaser','custom_thesis_thumb');