Welcome

If you have selected no avatar here, and no gravatar... then they're all different, unique (to that person) variations on a common theme.

If you have selected no avatar here, but have upload an image as your gravatar... then that's your avatar here.

If you have uploaded an avatar... then that takes priority over everything.

The idea is to encourage people to upload an avatar or gravatar that says something about them, and before that happens to still have a uniquely identifiable avatar in place to help with recognition when reading stuff.

what will they think of next?