You don't address why this is better than the gravatar options of a random image based on email hash:

identicon: a geometric pattern based on an email hash

monsterid: a generated 'monster' with different colors, faces, etc

wavatar: generated faces with differing features and backgrounds

retro: awesome generated, 8-bit arcade-style pixelated faces

from: http://www.gravatar.com/site/implement/images/

I can see why extra options are good, I'm just confused why you wouldn't compare it against this similar idea that people might be familiar with.