help with image resize

[Talguy]

I wrote and image resize script that keeps the same width height ratio as the orignal what I want to do is take the resized thumbnail and center a 100 by 100 pixel box on it so that my thumbnails are now 100 X 100. It would make my gallery look proportional and neatly in order. How would you go about doing this.

here is the code that I wrote

if($width > $height) {
  $thumbnail_width = 100;
  $thumbnail_height = (int)(100 * ($size[1] / $size[0]));
}
else {
  $thumbnail_width = (int)(100 * ($size[0] / $size[1]));
  $thumbnail_height = 100;
}

// Read the source file
$source_handle = $function_to_read($image_dir.'/'.$photo_category.'/images/'.$filename);

if($source_handle) {
  if($function_suffix == $photo_types['image/gif']) {
    // Create a blank image for gif files
    $destination_handle = imagecreate($thumnail_width, $thumbnail_height);
  }
  else {
    // Create a blank image for everything that is not a gif files
    $destination_handle = imagecreatetruecolor($thumbnail_width,$thumbnail_height);
  }
  // Let's create an blank image for the thumbnail
  // Take the blank file image(destinayion_handle)  

  // and fill it with the uploaded image($source_handle)
  imagecopyresized($destination_handle, $source_handle, 0, 0, 0, 0, $thumbnail_width, $thumbnail_height, $size[0], $size[1]);
}
else {
  echo 'Uploaded file does not exist <br />';
  $text = 'ERROR!! Could not find '.$image_dir.'/'.$photo_category.'/images/'.$filename.' to create the thumbnail for IP('.$_SERVER['REMOTE_ADDR'].')\n';
  fwrite($fileLog_handle, $text);
  exit();
}

// Let's save the thumbnail
$function_to_write($destination_handle, $image_dir.'/'.$photo_category.'/thumbs/tb_'.$filename);
imagedestroy($destination_handle);