title: navigation bar background did not change
body: <p>i have a navigation bar, i want to change it using image, i was implement this code</p>
  
  <pre><code>self.navBar.layer.contents = (id)[UIImage imageNamed:@"flower.png"].CGImage;
  </code></pre>
  
  <p>but the background is still didn't change. the hierarchy of navigation bar are :
  - UINavigationBar
  - UINavigationItem
  - UISegementControl</p>
  
  <p>i already set an outlet in every part in that hierarchy.. 
  is there something wrong???</p>