function swapPicture() {
    let bigImage = self.querySelector('.big-image picture');
    // insert the big image before the one clicked on
    this.parentNode.insertBefore(bigImage, this);
    // move the clicked image to the big image container
    self.querySelector('.big-image').appendChild(this);
    // swap event listeners
    this.removeEventListener('click', swapPicture);
    bigImage.addEventListener('click', swapPicture);

}

self.querySelectorAll('.gallery-images picture')
    .forEach((item, index) => {
        item.addEventListener('click', swapPicture);
    }
);