/// <summary>
 /// asp.net上传图片并生成缩略图
 /// </summary>
 /// <param name="upImage">HtmlInputFile控件</param>
 /// <param name="sSavePath">保存的路径,些为相对服务器路径的下的文件夹</param>
 /// <param name="sThumbExtension">缩略图的thumb</param>
 /// <param name="intThumbWidth">生成缩略图的宽度</param>
 /// <param name="intThumbHeight">生成缩略图的高度</param>
 /// <returns>缩略图名称</returns>
 public string UpLoadImage(HtmlInputFile upImage, string sSavePath, string sThumbExtension, int intThumbWidth, int intThumbHeight)
 {
  string sThumbFile = "";
  string sFilename = ""; 
  if (upImage.PostedFile != null)
  {
  HttpPostedFile myFile = upImage.PostedFile;
  int nFileLen = myFile.ContentLength;
  if (nFileLen == 0)
   return "没有选择上传图片";  
  //获取upImage选择文件的扩展名
  string extendName = System.IO.Path.GetExtension(myFile.FileName).ToLower();
  //判断是否为图片格式
  if (extendName != ".jpg" && extendName != ".jpge" && extendName != ".gif" && extendName != ".bmp" && extendName != ".png")
   return "图片格式不正确";
  
  byte[] myData = new Byte[nFileLen];
  myFile.InputStream.Read(myData, 0, nFileLen);
  sFilename = System.IO.Path.GetFileName(myFile.FileName);
  int file_append = 0;
  //检查当前文件夹下是否有同名图片,有则在文件名+1
  while (System.IO.File.Exists(System.Web.HttpContext.Current.Server.MapPath(sSavePath + sFilename)))
  {
   file_append++;
   sFilename = System.IO.Path.GetFileNameWithoutExtension(myFile.FileName)
   + file_append.ToString() + extendName;
  }
  System.IO.FileStream newFile
   = new System.IO.FileStream(System.Web.HttpContext.Current.Server.MapPath(sSavePath + sFilename),
   System.IO.FileMode.Create, System.IO.FileAccess.Write);
  newFile.Write(myData, 0, myData.Length);
  newFile.Close();
  //以上为上传原图
  try
  {
   //原图加载
   using (System.Drawing.Image sourceImage = System.Drawing.Image.FromFile(System.Web.HttpContext.Current.Server.MapPath(sSavePath + sFilename)))
   {
   //原图宽度和高度
   int width = sourceImage.Width;
   int height = sourceImage.Height;
   int smallWidth;
   int smallHeight;
   //获取第一张绘制图的大小,(比较 原图的宽/缩略图的宽 和 原图的高/缩略图的高)
   if (((decimal)width) / height <= ((decimal)intThumbWidth) / intThumbHeight)
   {
    smallWidth = intThumbWidth;
    smallHeight = intThumbWidth * height / width;
   }
   else
   {
    smallWidth = intThumbHeight * width / height;
    smallHeight = intThumbHeight;
   }
   //判断缩略图在当前文件夹下是否同名称文件存在
   file_append = 0;
   sThumbFile = sThumbExtension + System.IO.Path.GetFileNameWithoutExtension(myFile.FileName) + extendName;
   while (System.IO.File.Exists(System.Web.HttpContext.Current.Server.MapPath(sSavePath + sThumbFile)))
   {
    file_append++;
    sThumbFile = sThumbExtension + System.IO.Path.GetFileNameWithoutExtension(myFile.FileName) +
    file_append.ToString() + extendName;
   }
   //缩略图保存的绝对路径
   string smallImagePath = System.Web.HttpContext.Current.Server.MapPath(sSavePath) + sThumbFile;
   //新建一个图板,以最小等比例压缩大小绘制原图
   using (System.Drawing.Image bitmap = new System.Drawing.Bitmap(smallWidth, smallHeight))
   {
    //绘制中间图
    using (System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap))
    {
    //高清,平滑
    g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;
    g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
    g.Clear(Color.Black);
    g.DrawImage(
    sourceImage,
    new System.Drawing.Rectangle(0, 0, smallWidth, smallHeight),
    new System.Drawing.Rectangle(0, 0, width, height),
    System.Drawing.GraphicsUnit.Pixel
    );
    }
    //新建一个图板,以缩略图大小绘制中间图
    using (System.Drawing.Image bitmap1 = new System.Drawing.Bitmap(intThumbWidth, intThumbHeight))
    {
   //绘制缩略图 http://www.cnblogs.com/sosoft/
    using (System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap1))
    {
     //高清,平滑
     g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;
     g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
     g.Clear(Color.Black);
     int lwidth = (smallWidth - intThumbWidth) / 2;
     int bheight = (smallHeight - intThumbHeight) / 2;
     g.DrawImage(bitmap, new Rectangle(0, 0, intThumbWidth, intThumbHeight), lwidth, bheight, intThumbWidth, intThumbHeight, GraphicsUnit.Pixel);
     g.Dispose();
     bitmap1.Save(smallImagePath, System.Drawing.Imaging.ImageFormat.Jpeg);
    }
    }
   }
   }
  }
  catch
  {
   //出错则删除
   System.IO.File.Delete(System.Web.HttpContext.Current.Server.MapPath(sSavePath + sFilename));
   return "图片格式不正确";
  }
  //返回缩略图名称
  return sThumbFile;
  }
  return "没有选择图片";
 }

 <%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default2.aspx.cs" Inherits="Default2" %>
 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
 <html xmlns="http://www.w3.org/1999/xhtml" >
 <head runat="server">
 <title>图片上传-柯乐义</title>
 </head>
 <body>
 <form id="form1" runat="server">
 <div> 
  <input id="File1" runat="server" type="file" /></div><asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Button" />
 </form>
 </body>
 </html>

 protected void Button1_Click(object sender, EventArgs e)
 {
  string a = this.UpLoadImage(this.File1, "UpLoad/", "thumb_", 118, 118); 
 }