A implementação da lightbox em bootstrap requer alguns cuidados concretamente na localização dos ficheiros. Eis um vídeo que mostra o assunto:

 

Eis um código tipo em asp.net

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="lixo1.aspx.cs" Inherits="lixo1" %>

<!DOCTYPE html>

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">

    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
    <!-- Bootstrap CSS -->
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">
    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/ekko-lightbox/5.3.0/ekko-lightbox.css" />
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
        <div>
            <a href="/imagens_site/promoJillSander.png" data-toggle="lightbox">
                <img src="/imagens_site/promoJillSander.png" class="img-fluid" />
            </a>


            <script src="https://code.jquery.com/jquery-3.2.1.slim.min.js" integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN" crossorigin="anonymous"></script>
            <script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js" integrity="sha384-JZR6Spejh4U02d8jOt6vLEHfe/JQGiRRSQQxSfFWpi1MquVdAyjUar5+76PVCmYl" crossorigin="anonymous"></script>
            <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.9/umd/popper.min.js" integrity="sha384-ApNbgh9B+Y1QKtv3Rn7W3mgPxhU9K/ScQsAP7hUibX39j7fakFPskvXusvfa0b4Q" crossorigin="anonymous"></script>
            <script src="https://cdnjs.cloudflare.com/ajax/libs/ekko-lightbox/5.3.0/ekko-lightbox.js"></script>
            <script>
                $(document).on('click', '[data-toggle="lightbox"]', function (event) {
                    event.preventDefault();
                    $(this).ekkoLightbox();
                });

            </script>
        </div>
    </form>
</body>
</html>

 

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