在Ouroborus的评论后，我找到了解决方案：

// Create FANCYBOX

    // Show fancybox after click on a elemet of gallery
    gallery.find('a').on('click', function(event) {
        event.preventDefault();

        // Show fancybox
        overlay.appendTo('body');

        // Define variables
        var galleryImgWrapperHref = $(this).attr('href');

        // Show chosen image in fancybox
        image.attr('src', galleryImgWrapperHref);

        // Show loadin image before image is loaded
        loadingImg.appendTo('#fancybox__img-wrapper');

        // Image will show after it will be loaded, before loading image is showed
        image.on('load', function() {
            image.appendTo('#fancybox__img-wrapper');
            loadingImg.hide(); // Hide loading image afet image was loaded
        });

        // Make images slider for thumbnails from gallery
        if (thumbWrapper.length == 0) { // add thumbs just if it was not already added

            // Get id of current item
            var itemId = $('.item-detail').attr('id');

            // LOAD photos data to object
            $.ajax({
                url: '_inc/getPhotosFromDb.php',
                type: 'post',
                data: {
                    item_id: itemId
                }
            }).done(function(html) {
                galleryImgs = $(html);

                galleryImgs.each(function() {

                    var src = $(this).attr('src'),
                        href = src.replace('thumbs/','');
                        thumbWrapper = $('<a/>', {class: 'fancybox__img-thumb-wrapper'}),
                        thumbImg = $('<img>', {class: 'fancybox__img-thumb'});

                    // Add thumb image src and id from original image and href to a element from original e element in gallery
                    thumbWrapper.attr('href', href);
                    thumbImg.attr('src', src);
                    thumbWrapper.html(thumbImg); // Put image into thumbImgWrapper
                    thumbWrapper.appendTo('#fancybox__thumb-slider');

                    if ((thumbWrapper.attr('href').substring(thumbWrapper.attr('href').lastIndexOf("/") + 1) == image.attr('src').substring(image.attr('src').lastIndexOf("/") + 1))) {
                        thumbWrapper.addClass('img-active');
                    }
                })
            })
        }
        var bottomSliderAnchor = $(document).find('#fancybox__thumb-slider a'),
            imageNew = $(document).find('.fancybox__img'),
            bottomSlider = $(document).find('#fancybox__thumb-slider');

        bottomSliderAnchor.each(function() {
            var thumbWrapperNew = $(this);
            // If src of thumb and main image is the same then mark thumb as active
            if ((thumbWrapperNew.attr('href').substring(thumbWrapperNew.attr('href').lastIndexOf("/") + 1) == imageNew.attr('src').substring(imageNew.attr('src').lastIndexOf("/") + 1))) {
                thumbWrapperNew.addClass('img-active');
            }
        })
    });