压缩或放大图片

        /// <summary>
        /// 
        /// </summary>
        /// <param name="imgSource">源图片</param>
        /// <param name="destWidth">缩放后图片宽度</param>
        /// <param name="destHeight">缩放后图片高度</param>
        /// <returns></returns>
        public Image GetThumbnail(Image imgSource, int destWidth, int destHeight)
        {
            int sW = 0, sH = 0;
            int sWidth = imgSource.Width;
            int sHeight = imgSource.Height;

            // 按比例缩放
            if (sHeight > destHeight || sWidth > destWidth)
            {
                if ((sWidth * destHeight) > (sHeight * destWidth))
                {
                    sW = destWidth;
                    sH = (destWidth * sHeight) / sWidth;
                }
                else
                {
                    sH = destHeight;
                    sW = (sWidth * destHeight) / sHeight;
                }
            }
            else
            {
                sW = sWidth;
                sH = sHeight;
            }

            Bitmap outBmp = new Bitmap(destWidth, destHeight);
            Graphics g = Graphics.FromImage(outBmp);
            g.Clear(Color.White);

            // 设置画布的描绘质量
            g.CompositingQuality = CompositingQuality.HighQuality;
            g.SmoothingMode = SmoothingMode.HighQuality;
            g.InterpolationMode = InterpolationMode.HighQualityBicubic;

            g.DrawImage(imgSource, new Rectangle((destWidth - sW) / 2, (destHeight - sH) / 2, sW, sH), 0, 0, imgSource.Width, imgSource.Height, GraphicsUnit.Pixel);
            g.Dispose();

            return outBmp;
        }