title: button won't stay in UIControlStateHighlighted
body: <p>I'm trying to get a UIButton to change it's background image when the user clicks it.  I have the following code:</p>
  
  <pre><code>- (IBAction)indicatedButton:(id)sender
  {
  [sender setBackgroundImage:[UIImage imageNamed:@"blankTickBox.png"] forState:UIControlStateNormal];
  [sender setBackgroundImage:[UIImage imageNamed:@"completeTickBox.png"] forState:UIControlStateHighlighted];
  }
  </code></pre>
  
  <p>When I run the app on simulator, the image @"completeTickBox.png" flashes up for the second or so I have the mouse clicked on the button and then reverts to the original image.
  Any ideas?</p>