Preventing upscaling jaggies on flickr full size view

We don't know the width/height of flickr images and setting a default size frequently leads to ugly browser upscaling on portraits. Seems that the sensible thing here is to put 0 values in width and height and show them at the flickr defaults (I think this is max 500 pixels width or height). However this triggers the thumbnail view in flickr_guess_size(). Possibly a quick hack but one way of fixing this is to make the following small change in flickr.inc

function _image_ncck_flickr_guess_size($width, $height) {
  $max = max($width, $height);

  // new bit here
  if($max == 0)  { return null;  }

  foreach (array('s' => 75, 't' => 100, 'm' => 240, '' => 500, 'b' => 1024) as $size => $value) {
    if ($max <= $value) {
      return $size;
    }
  }
  return 'o';
}

N.B The thumbnails still seem to have the jaggies so this maybe just fixing a symptom.