private void MakeThumbImage(string path, string stpath, int width, int height, string mode)
    {
        System.Drawing.Image image = System.Drawing.Image.FromFile(path);
        int tw = width;
        int th = height;
        ///原始图片的宽度和高度
        int sw = image.Width;
        int sh = image.Height;
        int x = 0, y = 0;
        switch (mode)
        {
            case "HW": ///指定高宽缩放
                break;
            case "W":  ///指定图片的宽度，计算图片的高度
                th = image.Height * width / image.Width; break;
            case "H":  ///指定图片的高度，计算图片的宽度
                tw = image.Width * height / image.Height; break;
            case "CUT":
                ///计算缩略图的大小
                if ((double)tw / (double)th < (double)width / (double)height)
                {
                    sw = image.Width;
                    sh = image.Width * height / tw;
                    x = 0;
                    y = (image.Height - sh) / 2;
                }
                else
                {
                    sh = image.Height;
                    sw = image.Height * tw / th;
                    y = 0;
                    x = (image.Width - sw) / 2;
                }
                break;
            default: break;
        }

        ///创建bmp图片
        System.Drawing.Image bitmap = new System.Drawing.Bitmap(tw, th);
        ///创建Graphics对象g
        System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap);
        ///设置图像的插补模式
        g.InterpolationMode
            = System.Drawing.Drawing2D.InterpolationMode.High;
        ///设置图像的平滑程度
        g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
        ///清空画布，并以透明背景色填充
        g.Clear(System.Drawing.Color.Transparent);
        ///在指定位置并且按指定大小绘制原图片的指定部分
        g.DrawImage(image, new System.Drawing.Rectangle(0, 0, tw, th),
            new System.Drawing.Rectangle(x, y, sw, sh),
        System.Drawing.GraphicsUnit.Pixel);
        try
        {   ///采用jpg格式保存缩略图
            bitmap.Save(stpath, System.Drawing.Imaging.ImageFormat.Jpeg);
        }
        catch (Exception ex)
        {
            Response.Write(ex.Message); return;
        }
        finally
        {   ///释放资源
            image.Dispose();
            bitmap.Dispose();
            g.Dispose();
        }
    }


        MakeThumbImage(Server.MapPath("Images/xp.jpg"), Server.MapPath("Images/xp_01.jpg"), 80, 60, "CUT");