var initPhotoSwipeFromDOM = function(gallerySelector) {

// parse slide data (url, title, size ...) from DOM elements
// (children of gallerySelector)
var parseThumbnailElements = function(el) {
    var thumbElements = el.childNodes,
        numNodes = thumbElements.length,
        items = [],
        figureEl,
        linkEl,
        size,
        item;

    for(var i = 0; i < numNodes; i++) {

        figureEl = thumbElements[i]; // <figure> element

        // include only element nodes
        if(figureEl.nodeType !== 1) {
            continue;
        }

        linkEl = figureEl.children[0]; // <a> element

        size = linkEl.getAttribute('data-size').split('x');

        // create slide object
        item = {
            src: linkEl.getAttribute('href'),
            w: parseInt(size[0], 10),
            h: parseInt(size[1], 10)
        };



        if(figureEl.children.length > 1) {
            // <figcaption> content
            item.title = figureEl.children[1].innerHTML;
        }

        if(linkEl.children.length > 0) {
            // <img> thumbnail element, retrieving thumbnail url
            item.msrc = linkEl.children[0].getAttribute('src');
        }

        item.el = figureEl; // save link to element for getThumbBoundsFn
        items.push(item);
    }

    return items;
};

// find nearest parent element
var closest = function closest(el, fn) {
    return el && ( fn(el) ? el : closest(el.parentNode, fn) );
};

// triggers when user clicks on thumbnail
var onThumbnailsClick = function(e) {
    e = e || window.event;
    e.preventDefault ? e.preventDefault() : e.returnValue = false;

    var eTarget = e.target || e.srcElement;

    // find root element of slide
    var clickedListItem = closest(eTarget, function(el) {
        return (el.tagName && el.tagName.toUpperCase() === 'FIGURE');
    });

    if(!clickedListItem) {
        return;
    }

    // find index of clicked item by looping through all child nodes
    // alternatively, you may define index via data- attribute
    var clickedGallery = clickedListItem.parentNode,
        childNodes = clickedListItem.parentNode.childNodes,
        numChildNodes = childNodes.length,
        nodeIndex = 0,
        index;

    for (var i = 0; i < numChildNodes; i++) {
        if(childNodes[i].nodeType !== 1) {
            continue;
        }

        if(childNodes[i] === clickedListItem) {
            index = nodeIndex;
            break;
        }
        nodeIndex++;
    }



    if(index >= 0) {
        // open PhotoSwipe if valid index found
        openPhotoSwipe( index, clickedGallery );
    }
    return false;
};

// parse picture index and gallery index from URL (#&pid=1&gid=2)
var photoswipeParseHash = function() {
    var hash = window.location.hash.substring(1),
    params = {};

    if(hash.length < 5) {
        return params;
    }

    var vars = hash.split('&');
    for (var i = 0; i < vars.length; i++) {
        if(!vars[i]) {
            continue;
        }
        var pair = vars[i].split('=');
        if(pair.length < 2) {
            continue;
        }
        params[pair[0]] = pair[1];
    }

    if(params.gid) {
        params.gid = parseInt(params.gid, 10);
    }

    if(!params.hasOwnProperty('pid')) {
        return params;
    }
    params.pid = parseInt(params.pid, 10);
    return params;
};

var openPhotoSwipe = function(index, galleryElement, disableAnimation) {
    var pswpElement = document.querySelectorAll('.pswp')[0],
        gallery,
        options,
        items;

    items = parseThumbnailElements(galleryElement);

    // define options (if needed)
    options = {
        index: index,

        // define gallery index (for URL)
        galleryUID: galleryElement.getAttribute('data-pswp-uid'),

        getThumbBoundsFn: function(index) {
            // See Options -> getThumbBoundsFn section of documentation for more info
            var thumbnail = items[index].el.getElementsByTagName('img')[0], // find thumbnail
                pageYScroll = window.pageYOffset || document.documentElement.scrollTop,
                rect = thumbnail.getBoundingClientRect();

            return {x:rect.left, y:rect.top + pageYScroll, w:rect.width};
        }

    };

    if(disableAnimation) {
        options.showAnimationDuration = 0;
    }

    // Pass data to PhotoSwipe and initialize it
    gallery = new PhotoSwipe( pswpElement, PhotoSwipeUI_Default, items, options);
    gallery.init();
};

// loop through all gallery elements and bind events
var galleryElements = document.querySelectorAll( gallerySelector );

for(var i = 0, l = galleryElements.length; i < l; i++) {
    galleryElements[i].setAttribute('data-pswp-uid', i+1);
    galleryElements[i].onclick = onThumbnailsClick;
}

// Parse URL and open gallery if it contains #&pid=3&gid=1
var hashData = photoswipeParseHash();
if(hashData.pid > 0 && hashData.gid > 0) {
    openPhotoSwipe( hashData.pid - 1 ,  galleryElements[ hashData.gid - 1 ], true );
}
};

// execute above function
initPhotoSwipeFromDOM('.my-gallery');

<div itemscope itemtype="http://schema.org/ImageGallery">

    <figure itemprop="associatedMedia" itemscope itemtype="http://schema.org/ImageObject">
        <a href="large-image-1.jpg" itemprop="contentUrl">
           <img src="small-image.jpg" itemprop="thumbnail" alt="Image description" />
            <figcaption itemprop="caption description">Long image description 1</figcaption>
        </a>
    </figure>

    <figure itemprop="associatedMedia" itemscope itemtype="http://schema.org/ImageObject">
        <a href="large-image-2.jpg" itemprop="contentUrl">
            <figcaption itemprop="caption description">Long image description 2</figcaption>
        </a>
    </figure>

    ...

</div>