Added initial version.

js/002-swap-pictures.js

@@ -0,0 +1,17 @@
+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);
+    }
+);