catch_that_image & advanced custom fields

Hi,
im using advanced custom fields https://www.advancedcustomfields.com to display images, Im using a repeater field.
Im using the catch_that_image function to get the first image from each post — but id like to grab the first image from the ‘bilder_bxslider’ field. Ive benn trying a bit but without luck.

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( get_field('bilder_bxslider') ):
		while( has_sub_field('bilder_bxslider') ):
			$first_img = the_sub_field('bildsnurra');
		endwhile;
	endif;

  if(empty($first_img)) {
    $first_img = bloginfo( 'template_url' ).'/images/default.jpg'; 

  }
  return $first_img;
}