preload.js

var Preloader = function(sources, oncomplete, onload) {
    this.init(sources, oncomplete, onload);
}

Preloader.prototype = {
    init: function(sources, oncomplete, onload) {
        this.sources = sources;
        this.oncomplete = oncomplete
        this.onload = onload;
        this.images = [];
        this.counter = 0;
    },

    load: function() {
        var self = this;

        this.images = [];
        this.counter = 0;

        for (var i = 0; i < this.sources.length; i++) {
            var im = new Image();
            im.onload = function() {
                self.counter++;
                if (self.onload)
                    self.onload(this.src);
                if (self.counter == self.images.length)
                    self.oncomplete(self.sources);
            };
            im.src = this.sources[i];
            self.images.push(im);
        }
    }
};

function preload(images, complete, load) {
    var loader = new Preloader(images, complete, load);
    loader.load();
    return loader;
}