CSS

画像モーダル拡大

<a class="mbig" href=""><img src="●●●●.jpg" alt="" style="width:300px;" /></a>
<div class="modal">
   <div class="bigimg"><img src="" alt=""></div>
   <p class="close-btn"><a href="">✖</a></p>
</div>

CSS

/* モーダル画像の拡大 */
.modal {
      position: fixed;
      top: 0;
      left: 0;
      width: 100%;
      height: 100vh;
      background-color: rgba(0, 0, 0, 0.8);
      display: none;
		z-index:9999;
    }

    .bigimg {
      position: absolute;
      width: 80%;
      max-width: 800px;
      top: 80px;
      left: 0;
      right: 0;
      margin-right: auto;
      margin-left: auto;
    }

    .close-btn {
      color: #fff;
      font-size: 40px;
      position: absolute;
      right: 20px;
      top: 0;
    }

    .close-btn a {
      color: #fff;
      text-decoration: none;
    }

JS

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>
  <script>
    $(function() {
      $('a.mbig').click(function() {
        var imgSrc = $(this).children().attr('src');
        $('.bigimg').children().attr('src', imgSrc);
        $('.modal').fadeIn();
        $('body,html').css('overflow-y', 'hidden');
        return false
      });
      $('.close-btn').click(function() {
        $('.modal').fadeOut();
        $('body,html').css('overflow-y', 'visible');
        return false
      });
    });

  </script>