<?php
    function first_image() {
    global $post;

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

   if ($attached_image) {
          foreach ($attached_image as $attachment_id => $attachment) {
               set_post_thumbnail($post->ID, $attachment_id);
          }
      }
   }
   }

   echo first_image();
   ?>