Replaces all of the default identicon gravatar images on SO Chat with retro gravatar images. Click raw, and drag and drop in Extensions to install in Chrome.

retro-gravatar.user.js

// ==UserScript==
// @name          Retro Gravatar
// @author        Amaan Cheval
// @description	  Use retro gravatars instead of the indenticon ones
// @include       http://chat.stackexchange.com/rooms/*
// @include       http://chat.stackoverflow.com/rooms/*
// ==/UserScript==

var update = function () {
    [].forEach.call(document.querySelectorAll('.avatar > img'), function (elem) {
        elem.src = elem.src.replace('identicon', 'retro');
    });
};
document.body.addEventListener('DOMNodeInserted', update);
update();