How Do I Get A Full Image As Background Of My Header In Html/css?

When i choose a image to put as a background of my div i can't manage to make it look normal. The width and height of my header are made with em. Can someone explain me how i can g

Solution 1:

Check here, http://css-tricks.com/perfect-full-page-background-image/

DEMO http://jsfiddle.net/yeyene/PtEnY/3/

#yourHeader{
  float:left;
  width:120em;
  height:5em; 
  background: url(images/bg.jpg); 
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}