I edited your codepen a bit. So what I done:

• removed your on click handler
• added one handler for all clicks
• applied it only for your images
• added id for modal-img

I don't recommend you to use multiple ids with the same value. Would be better to use classes instead

// Get the modal
var modal = document.getElementById("myModal");

// Get the image and insert it inside the modal - use its "alt" text as a caption
// var img = document.getElementById("myImg");
var modalImg = document.getElementById("modal-img");
var captionText = document.getElementById("caption");
// img.onclick = function(){
//   modal.style.display = "block";
//   modalImg.src = this.src;
//   captionText.innerHTML = this.alt;
// }


document.addEventListener("click", (e) => {
  const elem = e.target;
  if (elem.id==="myImg") {
    modal.style.display = "block";
    modalImg.src = elem.dataset.biggerSrc || elem.src;
    captionText.innerHTML = elem.alt; 
  }
})

// Get the <span> element that closes the modal
var span = document.getElementsByClassName("close")[0];

// When the user clicks on <span> (x), close the modal
span.onclick = function() { 
  modal.style.display = "none";
}

#myImg {

  border-radius: 5px;
  cursor: pointer;
  transition: 0.3s;
}

#myImg:hover {opacity: 0.7;}


#myImg2 {

  border-radius: 5px;
  cursor: pointer;
  transition: 0.3s;
}

#myImg2:hover {opacity: 0.7;}

/* The Modal (background) */
.modal {
  display: none; /* Hidden by default */
  position: fixed; /* Stay in place */
  z-index: 1; /* Sit on top */
  padding-top: 100px; /* Location of the box */
  left: 0;
  top: 0;
  width: 100%; /* Full width */
  height: 100%; /* Full height */
  overflow: auto; /* Enable scroll if needed */
  background-color: rgb(0,0,0); /* Fallback color */
  background-color: rgba(0,0,0,0.9); /* Black w/ opacity */
}

/* Modal Content (image) */
.modal-content {
  margin: auto;
  display: block;
  width: 80%;
  max-width: 700px;
}

/* Caption of Modal Image */
#caption {
  margin: auto;
  display: block;
  width: 80%;
  max-width: 700px;
  text-align: center;
  color: #ccc;
  padding: 10px 0;
  height: 150px;
}

/* Add Animation */
.modal-content, #caption {  
  -webkit-animation-name: zoom;
  -webkit-animation-duration: 0.6s;
  animation-name: zoom;
  animation-duration: 0.6s;
}

@-webkit-keyframes zoom {
  from {-webkit-transform:scale(0)} 
  to {-webkit-transform:scale(1)}
}

@keyframes zoom {
  from {transform:scale(0)} 
  to {transform:scale(1)}
}

/* The Close Button */
.close {
  position: absolute;
  top: 15px;
  right: 35px;
  color: #f1f1f1;
  font-size: 40px;
  font-weight: bold;
  transition: 0.3s;
}

.close:hover,
.close:focus {
  color: #bbb;
  text-decoration: none;
  cursor: pointer;
}

/* 100% Image Width on Smaller Screens */
@media only screen and (max-width: 700px){
  .modal-content {
    width: 100%;
  }
}

<img style="width:30%" id="myImg" class="img-fluid" src="https://cdn.pixabay.com/photo/2015/04/23/22/00/tree-736885__340.jpg"
data-bigger-src="http://qnimate.com/wp-content/uploads/2014/03/images2.jpg"
>

 <img style="width:28%"  id="myImg" class="img-fluid" src="https://cdn.pixabay.com/photo/2015/06/19/21/24/the-road-815297__340.jpg">
 <img style="width:29%"  id="myImg" class="img-fluid" src="https://image.shutterstock.com/image-photo/mountains-during-sunset-beautiful-natural-260nw-407021107.jpg">

                                                    <!-- The Modal -->
<div id="myModal" class="modal">
  <span class="close">&times;</span>
  <img id="modal-img" class="modal-content"  src="https://encrypted-tbn0.gstatic.com/images?q=tbn%3AANd9GcQS2ol73JZj6-IqypxPZXYS3rRiPwKteoD8vezk9QsRdkjt3jEn&usqp=CAU">
    
  
  <div id="caption"></div>
</div>

Best solution that I have found:

Fisrt, the HTML changes:

Ex: Each image needs their own ID... very minor change to your html... (changes from w3school.com from that original file.

<div class="image"><img **id="myImg2"** src="images/pic02.jpg" alt="" /></div>
<div class="image"><img **id="myImg3"** src="images/pic03.jpg" alt="" /></div>

Original

    <div class="image"><img **id="myImg" src="images/pic01.jpg" alt="" /></div>

Second, CSS changes:

Copy the original "#myImg" and "#myImg:hover" attributes and paste right under .... adding the orinal new (number) after. Each image needs their unique "myImg"and their unique "#myImg:hover"

Ex:

#myImg2 {
    border-radius: 5px;
    cursor: pointer;
    transition: 0.3s;
}

#myImg3 {
    border-radius: 5px;
    cursor: pointer;
    transition: 0.3s;
}

#myImg2:hover {opacity: 0.7;}

#myImg3:hover {opacity: 0.7;}

Original

    #myImg {
        border-radius: 5px;
        cursor: pointer;
        transition: 0.3s;
    }

    #myImg:hover {opacity: 0.7;}

----

Javascript changes: Added the following: Add the var "variables" for each image and the "click" function for each image: --- Ex:

    var img2 = document.getElementById('myImg2');
    var img3 = document.getElementById('myImg3');

    //handle click for myImg2
    img2.onclick = function(){
        modal.style.display = "block";
        modalImg.src = this.src;
        captionText.innerHTML = this.alt;
    }

    //handle click for myImg3
    img3.onclick = function(){
        modal.style.display = "block";
        modalImg.src = this.src;
        captionText.innerHTML = this.alt;
    }

Orginal

<script>
    // Get the modal
var modal = document.getElementById('myModal');

// Get the image and insert it inside the modal - use its "alt" text as a caption
var img = document.getElementById('myImg');
var modalImg = document.getElementById("img01");
var img2 = document.getElementById('myImg2');
var img3 = document.getElementById('myImg3');
var captionText = document.getElementById("caption");
img.onclick = function(){
    modal.style.display = "block";
    modalImg.src = this.src;
    captionText.innerHTML = this.alt;
}

//handle click for myImg2
img2.onclick = function(){
    modal.style.display = "block";
    modalImg.src = this.src;
    captionText.innerHTML = this.alt;
}

//handle click for myImg3
img3.onclick = function(){
    modal.style.display = "block";
    modalImg.src = this.src;
    captionText.innerHTML = this.alt;
}

// Get the <span> element that closes the modal
var span = document.getElementsByClassName("close")[0];

// When the user clicks on <span> (x), close the modal
span.onclick = function() { 
  modal.style.display = "none";
}
    </script>

Here is an example of something very easy to understand:

<!DOCTYPE html>
<html>
<head>
<style>
#myImg {
    border-radius: 5px;
    cursor: pointer;
    transition: 0.3s;
}

#myImg2 {
    border-radius: 5px;
    cursor: pointer;
    transition: 0.3s;
}

#myImg3 {
    border-radius: 5px;
    cursor: pointer;
    transition: 0.3s;
}

#myImg:hover {opacity: 0.7;}

#myImg2:hover {opacity: 0.7;}

#myImg3:hover {opacity: 0.7;}

/* The Modal (background) */
.modal {
    display: none; /* Hidden by default */
    position: fixed; /* Stay in place */
    z-index: 1; /* Sit on top */
    padding-top: 100px; /* Location of the box */
    left: 0;
    top: 0;
    width: 100%; /* Full width */
    height: 100%; /* Full height */
    overflow: auto; /* Enable scroll if needed */
    background-color: rgb(0,0,0); /* Fallback color */
    background-color: rgba(0,0,0,0.9); /* Black w/ opacity */
}

/* Modal Content (image) */
.modal-content {
    margin: auto;
    display: block;
    width: 80%;
    max-width: 700px;
}

/* Caption of Modal Image */
#caption {
    margin: auto;
    display: block;
    width: 80%;
    max-width: 700px;
    text-align: center;
    color: #ccc;
    padding: 10px 0;
    height: 150px;
}

/* Add Animation */
.modal-content, #caption {    
    -webkit-animation-name: zoom;
    -webkit-animation-duration: 0.6s;
    animation-name: zoom;
    animation-duration: 0.6s;
}

@-webkit-keyframes zoom {
    from {-webkit-transform:scale(0)} 
    to {-webkit-transform:scale(1)}
}

@keyframes zoom {
    from {transform:scale(0)} 
    to {transform:scale(1)}
}

/* The Close Button */
.close {
    position: absolute;
    top: 15px;
    right: 35px;
    color: #f1f1f1;
    font-size: 40px;
    font-weight: bold;
    transition: 0.3s;
}

.close:hover,
.close:focus {
    color: #bbb;
    text-decoration: none;
    cursor: pointer;
}

/* 100% Image Width on Smaller Screens */
@media only screen and (max-width: 700px){
    .modal-content {
        width: 100%;
    }
}
</style>
</head>
<body>

<h2>Image Modal</h2>
<p>Guitarfetish.com </p>
<p>Gallery ideas</p>

<img id="myImg" src="img01.jpg" alt="XGP Tele Bodies" width="600" height="200">

<!-- The Modal -->
<div id="myModal" class="modal">
  <span class="close">&times;</span>
  <img class="modal-content" id="img01">
  <div id="caption"></div>
</div>

<img id="myImg2" src="img02.jpg" alt="Xaviere Guitars" width="600" height="200">

<img id="myImg3" src="img03.jpg" alt="Premium Guitar Cases" width="600" height="200">
<script>
// Get the modal
var modal = document.getElementById('myModal');

// Get the image and insert it inside the modal - use its "alt" text as a caption
var img = document.getElementById('myImg');
var img2 = document.getElementById('myImg2');
var img3 = document.getElementById('myImg3');
var modalImg = document.getElementById("img01");
var captionText = document.getElementById("caption");
img.onclick = function(){
    modal.style.display = "block";
    modalImg.src = this.src;
    captionText.innerHTML = this.alt;
}

//handle click for myImg2
img2.onclick = function(){
    modal.style.display = "block";
    modalImg.src = this.src;
    captionText.innerHTML = this.alt;
}

//handle click for myImg3
img3.onclick = function(){
    modal.style.display = "block";
    modalImg.src = this.src;
    captionText.innerHTML = this.alt;
}

// Get the <span> element that closes the modal
var span = document.getElementsByClassName("close")[0];

// When the user clicks on <span> (x), close the modal
span.onclick = function() { 
    modal.style.display = "none";
}
</script>

</body>
</html>

You are very close to your goal, you have 2 images

<img height="100" width="80" id="1" data-toggle="modal" data-target="#myModal" src='http://tympanus.net/Tutorials/CaptionHoverEffects/images/1.png' alt='Text dollar code part one.' />
<img height="100" width="80" id="2" data-toggle="modal" data-target="#myModal" src='http://tympanus.net/Tutorials/CaptionHoverEffects/images/2.png' alt='Text dollar code part one.' />

One Modal

<div id="myModal" class="modal fade" role="dialog">
    <div class="modal-dialog">
        <div class="modal-content">
            <div class="modal-body">
                <img class="img-responsive" src="" />
            </div>
            <div class="modal-footer">
                <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
            </div>
        </div>
    </div>
</div>

If you want to show image in modal with your approach using click function

$(document).ready(function () {
    $('img').on('click', function () {
        var image = $(this).attr('src');
        $('#myModal').on('show.bs.modal', function () {
            $(".img-responsive").attr("src", image);
        });
    });
});

Fiddle

Or you can use bootstrap modal event function only, less complicated and better approach (recommended solution)

$(document).ready(function () {
        $('#myModal').on('show.bs.modal', function (e) {
            var image = $(e.relatedTarget).attr('src');
            $(".img-responsive").attr("src", image);
        });
});

Fiddle

The span elements should be completely removed and its classes placed on the image elements themselves.

Also, you have a nested section element that isn't doing anything for you.

Lastly, do not use HTML heading elements (<h1>...<h6>) because of the way they style the text. Formatting is the job of CSS. Instead of headings, it is more appropriate to surround each image and its accompanying text with figure and figcaption elements.

img { 
  width:200px;
  border:1px solid black; /* This is only added for testing purposes*/
}

.thumbnail:hover {
   width: 500px;
   height:auto;
   position:relative;
   /* push image to the right by 1/2 the screen width and 1/2 the image width */
   margin-left:calc(50% - 250px);
}

<section id="main">
  <div class="inner">
    <div class="box alt">
      <div class="row 50% uniform">
         <div class="4u">
           <figure>
             <img src="https://pbs.twimg.com/profile_images/562466745340817408/_nIu8KHX.jpeg" alt="" class="thumbnail">
             <figcaption>Marble</figcaption>
           </figure>
         </div>

         <div class="4u">
           <figure>
             <img src="http://www.critterbabies.com/wp-content/gallery/kittens/cats-animals-kittens-background-us.jpg" alt="" class="thumbnail">
             <figcaption>Marble</figcaption>
           </figure>
         </div>
         
         <div class="4u">
           <figure>
             <img src="http://www.warrenphotographic.co.uk/photography/bigs/08482-Fluffy-ginger-female-kitten.jpg" alt="" class="thumbnail">
             <figcaption>Marble</figcaption>
           </figure>
         </div>
         
       </div>
     </div>
   </div>
 </section>