Facebook provides a type option for the picture field, for example, you can specify something like:
<img src="https://graph.facebook.com/xxx/picture?access_token=yyy&type=normal />
where the type parameter can be one of square, small, normal, or large for profile pictures or thumbnail, normal, album for album pictures.
Source: http://developers.facebook.com/docs/reference/api/user/ (under Connections section)
Edit: Added different options for album pictures