cabecalho

<div class="w3-content">
<img class="mySlides" src="banner.jpg"  style="width:100%" height="200px">

</div>
<nav class="navbar navbar-expand-lg navbar-dark bg-dark">
<a class="navbar-brand" href="#">Galeria</a>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNav" aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarNav">
<ul class="navbar-nav">
  <li class="nav-item">
    <a class="nav-link" href="galeria.php">Adicionar imagens <span class="sr-only"></span></a>
  </li>
  <li class="nav-item">
    <a class="nav-link" href="selecao.php">Ver Galeria</a>
  </li>

</ul>
</div>
</nav>