I haven't tried this with stream/feed photos, but the generally accepted way of doing this is:
http://graph.facebook.com/{ID of object}/picture
If you want the "large" version, you would do:
http://graph.facebook.com/{ID of object}/picture?type=large
I'm not 100% sure if this would work for an actual photo (instead of a user profile picture or page profile pic), but I have a hunch it will - the only caveat is that you obviously must have a logged in user that is authorized to view the photo (unless it's public).
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