缩略图生成代码：ThumbnailGenerator

    // 代码来自 http://blog.csdn.net/ojekleen/archive/2008/08/01/2754255.aspx
    // 有修改

    public class ThumbnailGenerator
    {
        /// <summary>
        /// 从源图像生成缩略图
        /// </summary>
        /// <param name="source"></param>
        /// <param name="thumbWidth"></param>
        /// <param name="thumbHeight"></param>
        /// <returns></returns>
        public Image GetThumbnail(Image source, int thumbWidth, int thumbHeight)
        {
            //原图宽度和高度 
            int width = source.Width;
            int height = source.Height;
            int smallWidth;
            int smallHeight;

            //获取第一张绘制图的大小,(比较 原图的宽/缩略图的宽  和 原图的高/缩略图的高) 
            if (((decimal)width) / height <= ((decimal)thumbWidth) / thumbHeight)
            {
                smallWidth = thumbWidth;
                smallHeight = thumbWidth * height / width;
            }
            else
            {
                smallWidth = thumbHeight * width / height;
                smallHeight = thumbHeight;
            }



            //新建一个图板,以最小等比例压缩大小绘制原图 
            using (Image tempBitmap = new Bitmap(smallWidth, smallHeight))
            {
                //绘制中间图 
                using (Graphics g = Graphics.FromImage(tempBitmap))
                {
                    //高清,平滑 
                    g.InterpolationMode = InterpolationMode.High;
                    g.SmoothingMode = SmoothingMode.HighQuality;
                    g.Clear(Color.Black);
                    g.DrawImage(source, new Rectangle(0, 0, smallWidth, smallHeight), new Rectangle(0, 0, width, height), GraphicsUnit.Pixel);
                }


                //新建一个图板,以缩略图大小绘制中间图 
                Image result = new Bitmap(thumbWidth, thumbHeight);
                //绘制缩略图 
                using (Graphics g = Graphics.FromImage(result))
                {
                    //高清,平滑 
                    g.InterpolationMode = InterpolationMode.High;
                    g.SmoothingMode = SmoothingMode.HighQuality;
                    g.Clear(Color.Black);
                    int lwidth = (smallWidth - thumbWidth) / 2;
                    int bheight = (smallHeight - thumbHeight) / 2;
                    g.DrawImage(tempBitmap, new Rectangle(0, 0, thumbWidth, thumbHeight), lwidth, bheight, thumbWidth, thumbHeight, GraphicsUnit.Pixel);
                }
                return result;
            }

        }

        /// <summary>
        /// 给定图片文件名，生成缩略图，并保存为文件。
        /// </summary>
        /// <param name="sourcePath"></param>
        /// <param name="destPath"></param>
        /// <param name="thumbWidth"></param>
        /// <param name="thumbHeight"></param>
        public void GenerateThumbnail(string sourcePath, string destPath, int thumbWidth, int thumbHeight)
        {
            if (!File.Exists(sourcePath))
            {
                throw new FileNotFoundException("sourcePath 指定的文件不存在。" + Environment.NewLine + sourcePath);
            }

            if (File.Exists(destPath))
            {
                throw new Exception("destPath 指定的文件已存在。" + Environment.NewLine + destPath);
            }

            //原图加载 
            using (Image sourceImage = Image.FromFile(sourcePath))
            {
                using (Image thumb = GetThumbnail(sourceImage, thumbWidth, thumbHeight))
                {
                    thumb.Save(destPath, ImageFormat.Jpeg);
                }
            }
        }

    }