void ResizeAndSave()
        {
            Bitmap bm = new Bitmap(PictureBox1.Image);
            Int32 x;
            Int32 y;
            int width = Val(x);
            int height = Val(y);
            Bitmap thumb = new Bitmap(width, height);
            Graphics g = Graphics.FromImage(thumb);
            g.InterpolationMode = Drawing2D.InterpolationMode.HighQualityBicubic;
            g.DrawImage(bm, new Rectangle(0, 0, width, height), new Rectangle(0, 0, bm.Width, bm.Height), GraphicsUnit.Pixel);
            g.Dispose();
            thumb.Save("C:\\newimage.bmp", System.Drawing.Imaging.ImageFormat.Bmp);
            bm.Dispose();
            thumb.Dispose();
            this.Close();
        }

------解决方案--------------------
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------解决方案--------------------

/// <summary>
       /// 生成缩略图
       /// </summary>
       /// <param name="originalImagePath">源图路径（物理路径）</param>
       /// <param name="thumbnailPath">缩略图路径（物理路径）</param>
       /// <param name="width">缩略图宽度</param>
       /// <param name="height">缩略图高度</param>
       /// <param name="mode">生成缩略图的方式</param>   
       public static void MakeThumbnail(string originalImagePath, string thumbnailPath, int width, int height, string mode)
       {
           Image originalImage = Image.FromFile(originalImagePath);

           int towidth = width;
           int toheight = height;

           int x = 0;
           int y = 0;
           int ow = originalImage.Width;
           int oh = originalImage.Height;

           switch (mode)
           {
               case "HW"://指定高宽缩放（可能变形）               
                   break;
               case "W"://指定宽，高按比例                   
                   toheight = originalImage.Height * width / originalImage.Width;
                   break;
               case "H"://指定高，宽按比例
                   towidth = originalImage.Width * height / originalImage.Height;
                   break;
               case "Cut"://指定高宽裁减（不变形）               
                   if ((double)originalImage.Width / (double)originalImage.Height > (double)towidth / (double)toheight)
                   {
                       oh = originalImage.Height;
                       ow = originalImage.Height * towidth / toheight;
                       y = 0;
                       x = (originalImage.Width - ow) / 2;
                   }
                   else
                   {
                       ow = originalImage.Width;
                       oh = originalImage.Width * height / towidth;
                       x = 0;
                       y = (originalImage.Height - oh) / 2;
                   }
                   break;
               default:
                   break;
           }

           //新建一个bmp图片
           Image bitmap = new System.Drawing.Bitmap(towidth, toheight);

           //新建一个画板
           Graphics g = System.Drawing.Graphics.FromImage(bitmap);

           //设置高质量插值法
           g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;

           //设置高质量,低速度呈现平滑程度
           g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;

           //清空画布并以透明背景色填充
           g.Clear(Color.Transparent);