That’s actually a bit of a tricky request.
Here’s one possible solution you might try out. It hides the original header image and instead sets up the container element for the header image with a background image using custom CSS (this overrides the Appearance > Header settings) and uses a CSS3 property called background-size to stretch the image to fit into the space (centered) no matter the width of the browser. I think it looks pretty cool, but not everyone likes how the background-size property works. Here’s what I came up with in case you’d like to try it out!
#page,
#masthead {
position: static;
}
.site-header-image > a > img {
display: none;
}
.site-header-image > a {
position: absolute;
left: 0;
top: 110px;
width: 100%;
height: 400px;
background: url(http://blogdottrendquarterdotcom.files.wordpress.com/2014/06/luxury-homes-geneva-switzerland-adelto_09.jpg) center no-repeat;
background-size: cover;
}
.main-navigation {
margin-top: 430px;
}
Kinda fun. :)