Взять комментированный контент из html

var text = document.getElementById('image-container').innerHTML.replace(/(<!--)|(-->)/g, '').trim(); 
 
document.getElementById('container').innerHTML= text;

<div id="image-container"> 
    <!-- 
    <img src="https://www.askideas.com/media/17/Cute-Ragdoll-Kittens.jpg" width="200" height="100"> 
    <img src="http://www.porjati.ru/uploads/posts/2015-05/thumbs/1431621727_1.jpg" width="200" height="100"> 
    --> 
</div> 
<div id="container"></div>

function enumDom() { 
  var div = document.getElementById('image-container'); 
  for (var i = 0; i < div.childNodes.length; i++) { 
    if (div.childNodes[i].nodeType == 8)  // NODE_COMMENT 
      console.log(div.childNodes[i].nodeValue); 
  } 
}

<div id="image-container"> 
    <!-- 
    <img src="http://lorempixel.com/200/100/animals/1" width="200" height="100"> 
    <img src="http://lorempixel.com/200/100/animals/2" width="200" height="100"> 
    --> 
</div> 
<div id="container"></div> 
<button onclick="enumDom()">Enum</button>

function showImage() { 
  var div = document.getElementById('image-container'); 
  for (var i = 0; i < div.children.length; i++) { 
    var img = div.children[i]; 
    img.src = img.attributes['data-src'].value; 
    img.style.display = 'inline'; 
  } 
}

#image-container img { 
  display: none; 
}

<div id="image-container"> 
    <img data-src="https://multator.ru/preview/uximrdanemqi" width="200" height="100"> 
</div> 
<div id="container"></div> 
<button onclick="showImage()">Show</button>