Author Avatar as default first image

I need little bit help and customization on my existng script at function.php for wordpress 3.5. Here’s what I have. As you can see here the default picture is none exist will be using

$first_img = $img_dir . '/images/post-default.jpg';

I would like to know if there would be possibility to just grab author’s posts picture. Please find below the whole code related to this function.
I really appreciated any recomendation…

////////////////////////////////////////////////////////////////////////////////
// Get Standard Post Image
////////////////////////////////////////////////////////////////////////////////
function get_post_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
    $img_dir = get_bloginfo('template_directory');
    $first_img = $img_dir . '/images/post-default.jpg';
  }
  return $first_img;
}