how to mask image using iphone sdk?

Question

I am using scrollview.There is a image in my scrollview.and my masking image is behind the scroll view.Now i am zooming this image and touch on mask button.But i cant get any output. i am very near to this but still not getting.

edit:- (UIImage*)imageByScalingAndCroppingForSize:(CGSize)targetSize{

newImage = nil; 
CGSize imageSize = oldImage.size;
CGFloat width = imageSize.width;
CGFloat height = imageSize.height;
CGFloat targetWidth = targetSize.width;
CGFloat targetHeight = targetSize.height;
CGFloat scaleFactor = 0.0;
CGFloat scaledWidth = targetWidth;
CGFloat scaledHeight = targetHeight;
CGPoint thumbnailPoint = CGPointMake(0.0,0.0);

if (CGSizeEqualToSize(imageSize, targetSize) == NO) {

  CGFloat widthFactor = targetWidth / width;
  CGFloat heightFactor = targetHeight / height;

  if (widthFactor > heightFactor)
  scaleFactor = widthFactor; // scale to fit height
  else
  scaleFactor = heightFactor; // scale to fit width
  scaledWidth = width * scaleFactor;
  scaledHeight = height * scaleFactor;

  if (widthFactor > heightFactor){
  thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5;
 }
  else
  if (widthFactor < heightFactor){
   thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5;
  }
}

UIGraphicsBeginImageContext(targetSize); 
CGRect thumbnailRect = CGRectZero;
thumbnailRect.origin = thumbnailPoint;
thumbnailRect.size.width = scaledWidth;
thumbnailRect.size.height = scaledHeight; 
[oldImage drawInRect:thumbnailRect]; 
newImage = UIGraphicsGetImageFromCurrentImageContext();

return newImage;

}

Edit:

In my application i hae implemented zooming functionality for image. Now i want to crop image or mask image for particular area of zooming image.