#header {
  display: block;
}
.backgroundImage{
    height: 800px;
    background-image: url('img/porche1.jpg');
    background-size: cover;
    background-position: center;
    background-repeat: no-repeat;
}

/* mobile : change l'image ET cache le header */
@media (max-width: 768px) {
  .backgroundImage{
    background-image: url('img/imgDilu/pneu-porche.jpg');
    background-size: cover;
    background-position: center;
    height: 850px;
  }

  #leftNav{
    display: none;
  }
  #rightNav{
    display: none;
  }
 #test{
    text-align: center;
    margin-left: 30px;
 }
}