image.js

var webpage = require('webpage'),
    q = require('sdk/core/promise');

var page = webpage.create();

var render = function(){
  page.render("~/google-image.png");
};

var modify = function(){
  var deferred = q.defer();

  // Can't find a way to get the window object and this makes the firefox to hang up
  page.window.document.write("<h1>Hey</h1>");

  if (page.window.readyState === 'complete') {
    deferred.resolve();
  }

  return deferred.promise;
};

page.open("http://google.com").then(modify).then(render);