gistfile1.php

/*
Plugin Name: Teaserimage
Plugin URI: 
Description: Allows to insert Teaserimages in your Theme
Author: Benjamin Birkenhake
Version: 0.1
Author URI: http://birkenhake.org
*/

add_filter( "the_content_feed", "teaserimage_addto_feed" );

/**
 * @param  $content Content of post
 * @return string
 */
function teaserimage_addto_feed($content){
    if(has_post_thumbnail()){
      $content = '<p>'.get_the_post_thumbnail(get_the_ID(), array(300, 300)).'</p>'."\n".$content;
    }    
    return $content;  
}