Hi there,
I can’t get you all the way there, but I can get you partially there.
First, change your HTML to this (no surrounding div, no link):
<img class="aligncenter size-full wp-image-270 featured-header-image" src="http://longhu.es.mialias.net/wp-content/uploads/2016/08/Imagen-portada-recortada.jpg" alt="Imagen-portada-recortada" width="1920" height="932">
Then add this CSS:
.featured-header-image {
position: relative;
top: -100px;
z-index: 1;
margin-bottom: -100px;
}
.secondary-navigation, .site-branding {
position: relative;
z-index: 2;
}