-(void)scrollViewDidZoom:(UIScrollView *)scrollView {
CGFloat xcenter = scrollView.center.x , ycenter = scrollView.center.y;
xcenter = scrollView.contentSize.width > scrollView.frame.size.width ? scrollView.contentSize.width/2 : xcenter;
ycenter = scrollView.contentSize.height > scrollView.frame.size.height ? scrollView.contentSize.height/2 : ycenter;
yourImageView.center = CGPointMake(xcenter, ycenter);
}