CSS: 100% ample amb fons

<style>
.divWithBgImage {
  width: 100%;
  height: 600px;
  background-image: url(image.jpg);
  background-repeat: no-repeat;
  background-size: 100%; //propotional resize
/*
  background-size: 100% 100%; //stretch resize
*/
}
</style>
<div class="divWithBgImage">
  some contents
</div>

Vist a https://stackoverflow.com/questions/12085881/css-100-width-and-background