Of Gravatars and Robohashes
Webmasters who use Gravatar to decorate comments with custom avatar images will be familiar with the d parameter of the Gravatar image request.
<img src="http://www.gravatar.com/avatars/8ca7425c8a9da807b9bf6934f10d59fa.jpg?d=monsterid" alt="" />You can specify values like monsterid, wavatar, identicon, or retro here and Gravatar will generate a unique avatar image corresponding to the MD5 hash of the user’s email address. You’ve seen these all around the web:
There are just a couple problems with these sets of generative graphics: