add: function(src, rel) {
    if (this.cache[src]) {
        return this.cache[src];
    }
    if (this.cache[rel]) {
    return this.cache[rel];
    }

    // The only "rel" value in this function is what you're passing to the "rel" parameter.
    alert( rel ); // take a look at the value

    // create a new image
    var image = new Image();

    // assign the src
    image.src = src;

    // assign the rel. Apparently we need to assign both the property and attribute separately
    image.rel = rel;
    image.setAttribute('rel',rel);

       // set the style of the image
    this.setStyle(image, {display: 'block'});

       // go ahead and return the image if it is already loaded
    if (image.complete && image.width) {
        return image;
    }
       // If we get this far, the image wasn't loaded yet, so we assign an "onload" handler
    image.onload = (function(scope) {
        return function() {
            scope.cache[src] = image;
        };
    })(this);

       // return the image
    return image;
},