Catch that image Function but with thumbnails

oops! My mistake,
I meant functions.php not plugins.
Sorry.
But heres what it looks like

function catch_that_image() {
  		global $post, $posts;
  		$first_img = '';
  		ob_start();
  		ob_end_clean();
  		$output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
  		$first_img = $matches [1] [0];

  		if(empty($first_img)){ //Defines a default image
  			$first_img = bloginfo('template_directory');
    		$first_img .= "/images/default.png";
  		}
  		return $first_img;
	}

and this is how I evoke the function in the themes code

<img src="<?php echo catch_that_image(); ?>" alt="<?php the_title(); ?>" width="770px" height="200px" class="featuredImage" />

[Please post code & markup between backticks or use the code button. Your posted code may now have been permanently damaged by the forum’s parser.]

So I’m wondering how I might get it to catch the thumbnail of the first image, rather than the entire image

Thanks Acrane,
But its not exactly what I mean

here’s my website, it may better illustrate what I mean
kidtrash.com
(The website is still largely broken so please ignore the mess)

You see in the “latest comic” window in the right sidebar,
I’d like for a cropped image of the image to appear there instead of
the whole thing. Sort of like a preview

Then I’d like for that image to link to the comics post.