/**//// <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);         

           //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(); 
           } 
       } 

4个重载方法,有直接返回Image对象的,有生成缩略图,并且保存到指定目录的!

using System.IO;
using System.Drawing;
using System.Drawing.Imaging;

/// <summary>
/// 图片处理类
/// 1、生成缩略图片或按照比例改变图片的大小和画质
/// 2、将生成的缩略图放到指定的目录下
/// </summary>
public class ImageClass
{
    public Image ResourceImage;
    private int ImageWidth;
    private int ImageHeight;

    public string ErrMessage;

    /// <summary>
    /// 类的构造函数
    /// </summary>
    /// <param name="ImageFileName">图片文件的全路径名称</param>
    public ImageClass(string ImageFileName)
    {
        ResourceImage=Image.FromFile(ImageFileName);
     ErrMessage="";
    }

    public bool ThumbnailCallback()
    {
     return false;
    }

    /// <summary>
    /// 生成缩略图重载方法1，返回缩略图的Image对象
    /// </summary>
    /// <param name="Width">缩略图的宽度</param>
    /// <param name="Height">缩略图的高度</param>
    /// <returns>缩略图的Image对象</returns>
    public Image GetReducedImage(int Width,int Height)
    {
     try
     {
      Image ReducedImage;

      Image.GetThumbnailImageAbort callb=new Image.GetThumbnailImageAbort(ThumbnailCallback); 
     
      ReducedImage=ResourceImage.GetThumbnailImage(Width,Height,callb,IntPtr.Zero);
  
      return ReducedImage;
     }
     catch(Exception e)
     {
      ErrMessage=e.Message; 
         return null;
     }
    }

    /// <summary>
    /// 生成缩略图重载方法2，将缩略图文件保存到指定的路径
    /// </summary>
    /// <param name="Width">缩略图的宽度</param>
    /// <param name="Height">缩略图的高度</param>
    /// <param name="targetFilePath">缩略图保存的全文件名，(带路径)，参数格式：D:\Images\filename.jpg</param>
    /// <returns>成功返回true，否则返回false</returns>
    public bool GetReducedImage(int Width,int Height,string targetFilePath)
    {
     try
     {
      Image ReducedImage;

      Image.GetThumbnailImageAbort callb=new Image.GetThumbnailImageAbort(ThumbnailCallback); 
     
      ReducedImage=ResourceImage.GetThumbnailImage(Width,Height,callb,IntPtr.Zero);
      ReducedImage.Save(@targetFilePath,ImageFormat.Jpeg);

      ReducedImage.Dispose(); 
  
      return true;
     }
     catch(Exception e)
     {
      ErrMessage=e.Message; 
      return false;
     }
    }

    /// <summary>
    /// 生成缩略图重载方法3，返回缩略图的Image对象
    /// </summary>
    /// <param name="Percent">缩略图的宽度百分比 如：需要百分之80，就填0.8</param>  
    /// <returns>缩略图的Image对象</returns>
    public Image GetReducedImage(double Percent)
    {
     try
     {
      Image ReducedImage;

      Image.GetThumbnailImageAbort callb=new Image.GetThumbnailImageAbort(ThumbnailCallback);

      ImageWidth=Convert.ToInt32(ResourceImage.Width*Percent);
      ImageHeight=Convert.ToInt32(ResourceImage.Width*Percent);
     
      ReducedImage=ResourceImage.GetThumbnailImage(ImageWidth,ImageHeight,callb,IntPtr.Zero);
  
      return ReducedImage;
     }
     catch(Exception e)
     {
      ErrMessage=e.Message; 
      return null;
     }
    }

    /// <summary>
    /// 生成缩略图重载方法4，返回缩略图的Image对象
    /// </summary>
    /// <param name="Percent">缩略图的宽度百分比 如：需要百分之80，就填0.8</param>  
    /// <param name="targetFilePath">缩略图保存的全文件名，(带路径)，参数格式：D:\Images\filename.jpg</param>
    /// <returns>成功返回true,否则返回false</returns>
    public bool GetReducedImage(double Percent,string targetFilePath)
    {
     try
     {
      Image ReducedImage;

      Image.GetThumbnailImageAbort callb=new Image.GetThumbnailImageAbort(ThumbnailCallback);

      ImageWidth=Convert.ToInt32(ResourceImage.Width*Percent);
      ImageHeight=Convert.ToInt32(ResourceImage.Width*Percent);
     
      ReducedImage=ResourceImage.GetThumbnailImage(ImageWidth,ImageHeight,callb,IntPtr.Zero);

      ReducedImage.Save(@targetFilePath,ImageFormat.Jpeg);

      ReducedImage.Dispose(); 
  
      return true;
     }
     catch(Exception e)
     {
      ErrMessage=e.Message; 
      return false;
     }
    }

}