Add thumbnail images to your WordPress posts

post_thumbnails.php

<?php
/*
Plugin Name: Post Thumbnails
Plugin URI: http://tarskitheme.com/
Description: Add thumbnail images to posts.
Author: Benedict Eastaugh
Version: 1.0
Author URI: http://extralogical.net/
*/

function add_thumbnail_to_post($the_content) {
  if (is_page() || is_single()) return $the_content;
  
  // Just add a custom field to your posts with the name 'thumbnail' and set
  // the value to the URI of the image you want to use.
  $thumbnail  = get_post_meta(get_the_ID(), 'thumbnail', true);
  $image      = '';
  
  if ($thumbnail) {
    $link  = get_permalink(get_the_ID());
    $image = sprintf('<a class="%s" href="%s"><img alt="%s" src="%s" /></a>',
                     'post-thumbnail imagelink',
                     $link,
                     '',
                     $thumbnail);
  }
  
  return $image . $the_content;
}

add_filter('the_content', 'add_thumbnail_to_post');

?>