C#生成缩略图的函数，可直接调用，代码非常简单，直接嵌入你的项目，调用该函数即可。

  /// <summary>
  /// 生成缩略图
 /// 转自：http://www.sharejs.com
  /// </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);      
  
      //在指定位置并且按指定大小绘制原图片的指定部分
      g.DrawImage(originalImage, new Rectangle(0, 0, towidth, toheight),
          new Rectangle(x, y, ow,oh),
          GraphicsUnit.Pixel);
  
      try
      {          
          //以jpg格式保存缩略图
          bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Jpeg);
      }
      catch(System.Exception e)
      {
          throw e;
      }
      finally
      {
          originalImage.Dispose();
          bitmap.Dispose();                      
          g.Dispose();
      }
  }