Using remote files with getimagesize($URL) never worked for me.  Except when I would grab files from the same server.  However, I developed some code with the help from the people here that does work.  If you are having problems give this function a shot:
 
function getimagesize_remote($image_url) {
   $handle = fopen ($image_url, "rb" );
   $contents = "";
   if ($handle) {
   do {
       $count += 1;
       $data = fread($handle, 8192);
       if (strlen($data) == 0) {
           break;
       }
   $contents .= $data;
   } while(true);
   } else { return false; }
   fclose ($handle);
 
   $im = ImageCreateFromString($contents);
   if (!$im) { return false; }
   $gis[0] = ImageSX($im);
   $gis[1] = ImageSY($im);
// array member 3 is used below to keep with current getimagesize standards
   $gis[3] = "width={$gis[0]} height={$gis[1]}";
   ImageDestroy($im);
   return $gis;
}