Adds a [post_thumbnail] shortcode for use with wordpress post thumbnails. Size can also be set using [post_thumbnail size=”XXXX”]. (You can call the function directly in your theme with <?php post_thumbnail('size=thumbnail');?>.)
Just insert the shortcode into any post’s content and when it will be replaced with the featured image.
This works only with the featured images introduced in WordPress 2.9.
For more information head to the full information page for WordPress Post Thumbnail Shortcode.