<div id="page">
  <div id="masthead">...</div>
  <div id="navigation">...</div>
  ...
</div>

#page {
  max-width: 1600px;
  min-width: 800px;
  width: 80%;
}

#masthead {
  background: url('path/to/image.jpg') no-repeat left top;
  height: 100px;
  width: auto;
}