shan-signature.php

<?php
/*
Plugin Name: Shan's Excellent Signature Plugin
*/
add_filter( 'the_content' , 'shan_add_signature' , 1 );
function shan_add_signature($text) {
   global $post;
   if($post->post_type == 'post') $text .= '<div class="signature"> <img src="' . get_stylesheet_directory_uri() . '/images/signature.jpg" alt="signature" /> </div>';
   return $text;
}