Sets an image associated with a post as featured if nothing is set

wp-auto-featured-image.php

<?php

function auto_featured_image() {
    global $post;

    if (!has_post_thumbnail($post->ID)) {
        $attached_image = get_children( "post_parent=$post->ID&amp;post_type=attachment&amp;post_mime_type=image&amp;numberposts=1&amp;order=ASC&amp;orderby=ID" );

      if ($attached_image) {
              foreach ($attached_image as $attachment_id => $attachment) {
                   set_post_thumbnail($post->ID, $attachment_id);
              }
         }
    }
}
// Use it temporary to generate all featured images
add_action('the_post', 'auto_featured_image');