Well I managed to fix it by adding:
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
Even works on the mobile landscape.
I have another question:
When I resize to tablet size the menu drops below the header, what css should I add to skip this menu and go straight to the little menu-toggle box (for phone-size)? Like this Colorlib page does. (no separate menu for tablet…)
Thanks