共有4种方法（也叫做函数），原本只是生成缩略图，生成后很不清楚，不好！所以我加入了优化缩略图的代码。只加在第二种方法里了。谁用的话可以比较一下，比其他的方法效果好太多了。

using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
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()
      {
      }

      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 ImageFileName, string targetFilePath)
      {
          Image ReducedImage = Image.FromFile(ImageFileName);
          //新建一个bmp图片
          System.Drawing.Image bitmap = new System.Drawing.Bitmap(Width, Height);

          //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(ReducedImage, new Rectangle(0, 0, Width, Height),
              new Rectangle(0, 0, ReducedImage.Width, ReducedImage.Height),
              GraphicsUnit.Pixel);

          try
          {
              //以jpg格式保存缩略图
              bitmap.Save(targetFilePath, System.Drawing.Imaging.ImageFormat.Jpeg);
              return true;
          }
          catch (System.Exception e)
          {
              throw e;
          }
          finally
          {
              ReducedImage.Dispose();
              bitmap.Dispose();
              g.Dispose();
          }
      }

      /// <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;
          }
      }

}

使用办法其中strBigImage代表了全路径以及文件名的要缩略图片地址,strThumbImage表示全路径以及文件名的生成文件名

ImageClass imgclass =new ImageClass();
            imgclass.GetReducedImage(180,180,strBigImage,strThumbImage);