Grab first image from Post in specific Category

Hi,

Thank you for this wonderful software!

I’m using the following code to grab the first image from the latest post
<?php echo get_first_image() ?>

which in turn calls this code

function get_first_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 no image displays show below image
if(empty($first_img)){ //Defines a default image
$first_img = "/images/default.jpg";
}
return $first_img;
}

I would like modify the code, so it grabs the first image from the latest post in a given Category.
I’ve tried for several hours but I’m not good enough with the coding, so I was hoping for some help from the forum. Thanks.