Image size option

Closes #11

Couple of things:

* I actually spent more time trying to get the tests to work but hey, at least I learnt something. :smile:

* Facebook's `normal` gets a 100x100 image and `small` gets a 50x50 image.
Meanwhile Twitter's `normal` gets a 48x48 image and `bigger` gets a 70x70 image. Oh well.