I’ve been asked this a few times, so here you go.
I use a WordPress theme that ties in commenter email addresses to a Gravatar image they set up themselves.
If you do not have a Gravatar you setup yourself, then this site randomly assigns one to you from a various set of generics.
If you would like to set up your own Gravatar that this site will display next to your name in the comments, simply visit the Gravatar website and follow the instructions to be found there! That’s where they all come from.
They’re just there for fun, but I’ll be straight with you… I love seeing what people choose as their own Gravatar, their visual depiction of themselves.