title: how to add watermark on a exist image
body: <p>I found some code as follows:</p>
  
  <pre><code> UIGraphicsBeginImageContext(CGSizeMake(320, 480));
  // This is where we resize captured image
  [(UIImage *)[info objectForKey:UIImagePickerControllerOriginalImage] drawInRect:CGRectMake(0, 0, 320, 480)];
  // And add the watermark on top of it
  [[UIImage imageNamed:@"Watermark.png"] drawAtPoint:CGPointMake(0, 0) blendMode:kCGBlendModeNormal alpha:WATERMARK_ALPHA];
  // Save the results directly to the image view property
  imageView.image = UIGraphicsGetImageFromCurrentImageContext();
  UIGraphicsEndImageContext();
  </code></pre>
  
  <p>but I am not sure whether it's the best way.</p>