preloadImage(imgUrls, callback).js

/*
 * Copyright (C) 2012 Eike Send
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to
 * deal in the Software without restriction, including without limitation the
 * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
 * sell copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 * IN THE SOFTWARE.
 */

function preloadImages(imgUrls, callback) {
  "use strict";
  var images = [];
  var numLoaded = 0;
  imgUrls = Object.prototype.toString.call(imgUrls) === '[object Array]'
  	? imgUrls
  	: [imgUrls];
  var handleLoad = function() {
    numLoaded++;
    // Some browsers fire onload events whenever an animated
    // GIF restarts a loop, so remove the on* handlers now.
    this.onabort = this.onerror = this.onload = null;
    if (callback && numLoaded === imgUrls.length) {
      callback(images);
    }
  };
  var image;
  for (var i = 0; i < imgUrls.length; i++) {
    image = images[i] = new Image();
    image.onabort = image.onerror = image.onload = handleLoad;
    image.src = imgUrls[i];
    if (image.complete) {
      // Image is likely cached. Fire the handler right away.
      image.onabort = image.onerror = image.onload = null;
      handleLoad.call(image);
    }
  }
}

Example: http://jsfiddle.net/steveluscher/ggwqvfro/
Related: http://stackoverflow.com/a/8032110/802047