Image modals on multi-tab page

I want to create a multi-tabs page with image modal on each page. I used code from w3school.com. The modal only shows on the first page but not others. No matter what image I click, the modal with that image will show on the first page. I am new to web design, so I prefer to use css and simple javascript instead of stylesheet.

Can anybody help me figure out how to make the modal show on the correct page?

My html

<button class="tablink" onclick="openPage('Home', this, 'red')">Home</button>
<button class="tablink" onclick="openPage('News', this, 'green')" id="defaultOpen">News</button>
<button class="tablink" onclick="openPage('Contact', this, 'blue')">Contact</button>
<button class="tablink" onclick="openPage('About', this, 'orange')">About</button>

<div id="Home" class="tabcontent">
<h2>Image Modal</h2>
<p>In this example, we use CSS to create a modal (dialog box) that is hidden by default.</p>
<p>We use JavaScript to trigger the modal and to display the current image inside the modal when it is clicked on. Also note that we use the value from the image's "alt" attribute as an image caption text inside the modal.</p>

<img id="myImg" src="img_nature.jpg" alt="Snow" style="width:100%;max-width:300px">

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

<div id="News" class="tabcontent">
<h2>Image Modal</h2>
<p>In this example, we use CSS to create a modal (dialog box) that is hidden by default.</p>
<p>We use JavaScript to trigger the modal and to display the current image inside the modal when it is clicked on. Also note that we use the value from the image's "alt" attribute as an image caption text inside the modal.</p>

<img id="myImg" src="img_snow.jpg" alt="Snow" style="width:100%;max-width:300px">

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

<div id="Contact" class="tabcontent">
<h2>Image Modal</h2>
<p>In this example, we use CSS to create a modal (dialog box) that is hidden by default.</p>
<p>We use JavaScript to trigger the modal and to display the current image inside the modal when it is clicked on. Also note that we use the value from the image's "alt" attribute as an image caption text inside the modal.</p>

<img id="myImg" src="img_snow.jpg" alt="Snow" style="width:100%;max-width:300px">

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

<div id="About" class="tabcontent">
  <h3>About</h3>
  <p>Who we are and what we do.</p>
</div>

My css

* {box-sizing: border-box}

/* Set height of body and the document to 100% */
body, html {
  height: 100%;
  margin: 0;
  font-family: Arial;
}

/* Style tab links */
.tablink {
  background-color: #555;
  color: white;
  float: left;
  border: none;
  outline: none;
  cursor: pointer;
  padding: 14px 16px;
  font-size: 17px;
  width: 25%;
}

.tablink:hover {
  background-color: #777;
}

/* Style the tab content (and add height:100% for full page content) */
.tabcontent {
  color: white;
  display: none;
  padding: 100px 20px;
  height: 100%;
}

#Home {background-color: red;}
#News {background-color: green;}
#Contact {background-color: blue;}
#About {background-color: orange;}

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

#myImg: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%;
  }
}

script

<script>
function openPage(pageName,elmnt,color) {
  var i, tabcontent, tablinks;
  tabcontent = document.getElementsByClassName("tabcontent");
  for (i = 0; i < tabcontent.length; i++) {
    tabcontent[i].style.display = "none";
  }
  tablinks = document.getElementsByClassName("tablink");
  for (i = 0; i < tablinks.length; i++) {
    tablinks[i].style.backgroundColor = "";
  }
  document.getElementById(pageName).style.display = "block";
  elmnt.style.backgroundColor = color;
}

// Get the element with id="defaultOpen" and click on it
document.getElementById("defaultOpen").click();


// 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 captionText = document.getElementById("caption");
img.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>