方法1

HttpFileCollection files   = HttpContext.Current.Request.Files;
//我设置的大小不超过A4纸的缩略图
int newWidth=620;
int newHeight=880;
System.Drawing.Image img = null;
            
for(int iFile = 0; iFile < files.Count; iFile++)
{
     ///检查文件扩展名字
     HttpPostedFile postedFile = files[iFile];
     string fileName, fileExtension;
     fileName = System.IO.Path.GetFileName(postedFile.FileName);
     if (fileName != "")
     {
         fileExtension = System.IO.Path.GetExtension(fileName);
         string name=DateTime.Now.ToString("yyyyMMddHHmmssffffff")+iFile+fileExtension;
         string filePath=System.Web.HttpContext.Current.Request.MapPath(@"UpdatePic/") + name;
                        
         if (fileExtension.ToLower()==".jpg" || fileExtension.ToLower()==".gif" || fileExtension.ToLower()==".png")
         {
             //图片大小限制在1K到2M之间
             if(postedFile.ContentLength>=1024 && postedFile.ContentLength<=20480000 && postedFile.ContentType.ToString().ToLower().IndexOf("image") >= 0)
             {
                 img = System.Drawing.Image.FromStream(postedFile.InputStream);
                 int iWidth   = img.Width;
                 int iHeight = img.Height;
                 //如果宽和高都超过最大限制
                 if(iWidth > newWidth && iHeight > newHeight)
                 {
                     //判断下是宽更长还是高更长，以便以它做标准
                     if((double)iHeight/(double)iWidth > (double)newHeight/(double)newWidth)
                     {
                         //如果是高更长，设置高为最大限制值，宽等比例缩小
                         newWidth=(int)((double)iWidth/(double)iHeight * newHeight);
                     }
                     else
                     {
                         //如果是宽更长，设置宽为最大限制值，高等比例缩小
                         newHeight=(int)((double)iHeight/(double)iWidth * newWidth);
                     }
                 }
                 else if(iWidth > newWidth && iHeight <= newHeight)
                 {
                     //如果只是宽超过限制，设置宽为最大值，高等比例缩小
                     newHeight=(int)((double)iHeight/(double)iWidth * newWidth);
                 }
                 else if(iWidth <= newWidth && iHeight > newHeight)
                 {
                     //如果只是高超过限制，设置高为最大值，宽等比例缩小
                     newWidth=(int)((double)iWidth/(double)iHeight * newHeight);
                 }
                 else
                 {
                     //如果都没超过现在用原来的大小
                     newWidth=iWidth;
                     newHeight=iHeight;
                 }
                 //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);
                 //按指定大小绘制原图片
                 g.DrawImage(img, new Rectangle(0, 0, width, height));

                 try
                 {            
                     //以jpg格式保存缩略图，图片质量更高
                     bitmap.Save(imgPath, System.Drawing.Imaging.ImageFormat.Jpeg);
                 }
                 catch(System.Exception e)
                 {
                     throw e;
                 }
                 finally
                 {
                     img.Dispose();
                     bitmap.Dispose();                        
                     g.Dispose();
                 }
             }
         }
     }
}
方法二。

private void btnUploadPicture_Click(object sender, System.EventArgs e)
{
   //检查上传文件的格式是否有效
   if(this.UploadFile.PostedFile.ContentType.ToLower().IndexOf("image") < 0)
   {
     Response.Write("上传图片格式无效！");
     return;
   }

   //生成原图
   Byte[] oFileByte = new byte[this.UploadFile.PostedFile.ContentLength];
   System.IO.Stream oStream = this.UploadFile.PostedFile.InputStream;
   System.Drawing.Image oImage = System.Drawing.Image.FromStream(oStream);

   int oWidth = oImage.Width; //原图宽度
   int oHeight = oImage.Height; //原图高度
   int tWidth = 100; //设置缩略图初始宽度
   int tHeight = 100; //设置缩略图初始高度

   //按比例计算出缩略图的宽度和高度
   if(oWidth >= oHeight)
   {
     tHeight = (int)Math.Floor(Convert.ToDouble(oHeight) * (Convert.ToDouble(tWidth) / Convert.ToDouble(oWidth)));
   }
   else
   {
     tWidth = (int)Math.Floor(Convert.ToDouble(oWidth) * (Convert.ToDouble(tHeight) / Convert.ToDouble(oHeight)));
   }

   //生成缩略原图
   Bitmap tImage = new Bitmap(tWidth,tHeight);
   Graphics g = Graphics.FromImage(tImage);
   g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High; //设置高质量插值法
   g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;//设置高质量,低速度呈现平滑程度
   g.Clear(Color.Transparent); //清空画布并以透明背景色填充
   g.DrawImage(oImage,new Rectangle(0,0,tWidth,tHeight),new Rectangle(0,0,oWidth,oHeight),GraphicsUnit.Pixel);

   string oFullName = Server.MapPath(".") "/" "o" DateTime.Now.ToShortDateString().Replace("-","") DateTime.Now.Hour.ToString() DateTime.Now.Minute.ToString() DateTime.Now.Second.ToString() DateTime.Now.Millisecond.ToString() ".jpg"; //保存原图的物理路径
   string tFullName = Server.MapPath(".") "/" "t" DateTime.Now.ToShortDateString().Replace("-","") DateTime.Now.Hour.ToString() DateTime.Now.Minute.ToString() DateTime.Now.Second.ToString() DateTime.Now.Millisecond.ToString() ".jpg"; //保存缩略图的物理路径

   try
   {
     //以JPG格式保存图片
     oImage.Save(oFullName,System.Drawing.Imaging.ImageFormat.Jpeg);
     tImage.Save(tFullName,System.Drawing.Imaging.ImageFormat.Jpeg);
   }
   catch(Exception ex)
   {
     throw ex;
   }
   finally
   {
     //释放资源
     oImage.Dispose();
     g.Dispose();
     tImage.Dispose();
   }
}