foomarks

Solved, apply a filter to the_content:

function catch_that_image() {
global $post, $posts;
$first_img = '';
ob_start();
ob_end_clean();
$transformed_content = apply_filters('the_content',$post->post_content);
$output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $transformed_content, $matches);
$first_img = $matches [1] [0];
if(empty($first_img)){ //Defines a default image
$first_img = "/images/default.jpg";
}
return $first_img;
}