base.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Flask</title>
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.4.0/css/bootstrap.min.css">
</head>
<body>
    <nav class="navbar navbar-inverse">
      <div class="container-fluid">
        <div class="navbar-header">
          <a class="navbar-brand" href="#">WebSiteName</a>
        </div>
        <ul class="nav navbar-nav">
          <li class="active"><a href="#">Home</a></li>
          <li><a href="#">Page 1</a></li>
          <li><a href="#">Page 2</a></li>
          <li><a href="#">Page 3</a></li>
        </ul>
      </div>
    </nav>

    <center>
        <img src="{{ url_for('static',
        filename='img/download.jpg') }}" alt="Image"/>
    </center>

    <h1>Welcome to Flask.</h1>

    {% block content %}
    {% endblock %}

</body>
</html>


!just the base file.