gistfile1.js

$.currActive;
$.imageCount;

$.fn.preCacheImageFromThumb = function () {
    return $(this).each(function () {
        var img = new Image();
        img.src = $(this).attr('href');
    });
};


$.removeCurrActive = function () {
    if ($.currActive) {
        $.currActive.find('.staging_thumb-numb-indicator').removeClass('active');
        $.currActive = null;
    }
};

$.setCurrActive = function (currActive) {
    $.removeCurrActive();
    $.currActive = $(currActive);
    $.currActive.find('.staging_thumb-numb-indicator').addClass('active');
    $('#staging_main-image').attr('src', $.currActive.attr('href'));
};

$(document).ready(function () {
    $.imageCount = $('.staging_thumb-link').length;
    
    $.currActive = $('.staging_thumb-numb-indicator.active').closest('a');
    
    $('.staging_thumb-link').preCacheImageFromThumb().click(function (evt) {
        evt.preventDefault();
        $.setCurrActive(this);
    });
    
    $('.staging_secondary-photo-nav').click(function(evt) {
        evt.preventDefault();
        var currIndex = parseInt($.currActive.attr('id').substr("staging_thumb-link-".length), 10);
        var delta = parseInt($(this).attr('rel'), 10);
        var nextIndex = (currIndex + ($.imageCount + delta)) % $.imageCount;
        console.log(nextIndex);
        $.setCurrActive($("#staging_thumb-link-" + nextIndex));
    });

});