Test that a valid image is actually being loaded.

gistfile1.txt

  public function checkForImage($url) {

  $ch  = curl_init($url);
  curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
  curl_exec($ch);

  // get the content type
  $mime_type = curl_getinfo($ch, CURLINFO_CONTENT_TYPE);
  if (strpos($mime_type, 'image/') === FALSE) {
    throw new Exception (sprintf('%s did not return an image', $url));
  
  }

Thanks. This is exactly what I needed.