Hi Ke Li, The 328 and 160x160 are used by the Web application to show thumbnails. But we also have an Android client which downloads different size (I think 640x480). And there may be future clients which download different sizes.

Is it possible to change this like:

```
def related_keys(for_key)
  self['photo_keys'].keys.select { |check_key| check_key.starts_with? for_key
end
```

That's a rough idea, I'm not sure whether we need to use `self['photo_keys']` or `self['_attachments']`, but can you please think on those lines too?