preload.js

var oldPreload = {

    myImages: [],                // stores image url paths
	
    counter: 0,                  // keeps count of all loaded images
	
    intervalId: null,            // keeps setInterval pointer id
	
    imageFolder: [],             // stores images objects 
	
    loaded: [],                  // keeps a boolean value for all images, true if loaded, false otherwise.

    onComplete: function() {},                             // method called when preloading is complete
	
    startLoading: function () {                            // initialize preloading
	    var i, totalImages = this.myImages.length;
		
        for (i = 0; i < totalImages; i += 1) {             // loop thrugh all image urls
            this.imageFolder[i] = new Image()              // store image objects in temp folder
            this.imageFolder[i].src = this.myImages[i];    // define src attribute
			this.loaded[i] = false;                        // assume none has been loaded
        }
		
        this.intervalId = setInterval(this.checkLoad, 10); // check every 10 mlliseconds to see if images have loaded 
     },                     

    checkLoad: function () {
	    var i,  totalImages = oldPreload.myImages.length;
		
        if (oldPreload.counter === totalImages) {  // all images have been loaded
            clearInterval(oldPreload.intervalId); //  stop checking, clear intervals
			oldPreload.onComplete();              //  call the onComplete method
            return;                               // exit
        }
		
        for (i = 0; i < totalImages; i += 1) {   // loop through all image urls
            if (!oldPreload.loaded[i] && oldPreload.imageFolder[i].complete) { // an has finished loading and is not yet marked
                oldPreload.loaded[i] = true;                                   // mark it as loaded 
				try { console.log(oldPreload.imageFolder[i].src + ' - loading complete');} // show result in firebug console
                catch(e){}
                oldPreload.counter += 1;  // keep counting. one more image has been loaded 
            }	
        }	
    }
};