下拉放大头部图片

自定义view里面方imageView，放大ImageView的height

// ImageView的contentMode要设置成这种，这样会让宽度随着高度一起放大
  self.headImageView.contentMode = UIViewContentModeScaleAspectFill;

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    CGPoint point = scrollView.contentOffset;
    if (point.y <= 0) {
        // 205是图片的原始高度
        CGRect rect = ImageView.frame;
        rect.origin.y = point.y;
        rect.size.height = 205-point.y;
        ImageView.frame = rect;
    }else {
        ImageView.frame = CGRectMake(0, 0, SCREEN_WIDTH, 205);
    }
}