<?php

function my_related_posts_thumbnail($width = 120,$height = 160){
	global $post;
	$permalink = get_permalink($post->ID);
	$title = $post->post_title;
	$post_img = '';
	ob_start();
	ob_end_clean();
	$output = preg_match('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);   
    $post_img_src = $matches [1]; 
	if(empty($post_img_src)){
		$post_img = '<a href="' . $permalink . '" title="'.$post->post_title.'"><img src="'.get_bloginfo("template_url").'/images/default_thumb.jpg" alt="'.$post->post_title.'" />'.$post->post_title.'</a>';
	}else{
		$post_img = '<a href="' . $permalink . '" title="'.$post->post_title.'"><img src="'.$post_img_src.'"  alt="'.$post->post_title.'" />'.$post->post_title.'</a>'; 
	}
	echo $post_img;
}

function my_related_posts($post_num = 4) {
	global $post;
	$exclude_id = $post->ID;
	$i = 0;
	$cats = ''; 
	foreach ( get_the_category() as $cat ) $cats .= $cat->cat_ID . ',';
	$args = array(
		'category__in' => explode(',', $cats),
		'post__not_in' => explode(',', $exclude_id),
		'caller_get_posts' => 1,
		'orderby' => 'rand',
		'posts_per_page' => $post_num - $i
	);
	query_posts($args);
	while( have_posts() ) { the_post(); 
		echo "<li>" . my_related_posts_thumbnail() . "</li>";
		$i++;
	} 
	wp_reset_query();
	if ( $i == 0 )  echo '<li>No related posts!</li>';
}

?>