You check the Graph object for this id,
graph.facebook.com/photoID
For example, a photo from facebook.com/facebook
http:
This will return a JSON response for which you can select the desired source size.
See http://developers.facebook.com/docs/reference/api/photo/ for more details.
For example, in the PHP SDK
$a_photo = $facebook->api('/427100966728'); <img src=<?php echo $a_photo['picture'] ?> />
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