public void MakeThumbnail(string imgPath_old, int width, int height)
{
   System.Drawing.Image img = System.Drawing.Image.FromStream(new System.IO.MemoryStream(System.IO.File.ReadAllBytes(imgPath_old)));
 
  int towidth = width; int toheight = height;
  int x = 0; int y = 0; int ow = img.Width;
  int oh = img.Height;
  // 按值较大的进行等比缩放（不变形） 
  if ((double)img.Width / (double)towidth < (double)img.Height / (double)toheight)
  {
    toheight = height;
    towidth = img.Width * height / img.Height;
  }
  else
  {
    towidth = width;
    toheight = img.Height * width / img.Width;
  }
  //新建一个bmp图片
  System.Drawing.Image bitmap = new System.Drawing.Bitmap(towidth, toheight);
  //新建一个画板
  System.Drawing.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(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(img, new System.Drawing.Rectangle(0, 0, towidth, toheight),
  new System.Drawing.Rectangle(x, y, ow, oh),
  System.Drawing.GraphicsUnit.Pixel);
  String newFile = Server.MapPath("~/small_" + Path.GetFileName(imgPath_old));
  bitmap.Save(newFile , System.Drawing.Imaging.ImageFormat.Jpeg);
  bitmap.Dispose();
  img.Dispose();
  g.Dispose();
}
 
protected void Page_Load(object sender, EventArgs e)
{
  MakeThumbnail(Server.MapPath("~/aaa.jpg"), 100, 100);
}