#header {
    width: 100%;
    background: url('/Index_archivos') no-repeat;
    position: relative;
    height: 658px;
    background-position-x: 172px;
}
body {
  height: 500px;
  background-image: url("https://runner.franox.es/img/Picu_Urriellu.jpg");
  background-repeat:no-repeat;
  background-attachment:fixed;
  background-position-x:center;
  background-position-y:center;
  background-size: cover;
}