CSS:

#header {
        height:120px;
        border-bottom:5px solid black;
}


#content  {
        background-image: url('../img/background.jpg');
        background-repeat: no-repeat;
        background-size:cover;
        width:100%;
        height:100%;
}

HTML:
<div id="header"></div>

<div id="content">
</div>