//ajax保存数据，后台方法里实现此方法 
function savedata() {　 
    filename = document.getelementbyid("idfile").value; 
    result =test_test_aspx.savedata(filename).value; 
    if (result) { 
      alert("保存成功！");      
    } 
    return false; 
  }  
//实现预览功能 
  function drawimage(imgd) { 
    var prew = 118; 
    var preh = 118; 
    var image = new image(); 
    image.src = imgd.src; 
    if (image.width > 0 && image.height > 0) { 
      flag = true; 
      if (image.width / image.height >= prew/ preh) { 
        if (image.width > prew) { 
          imgd.width = prew; 
          imgd.height = (image.height * prew) / image.width; 
        } 
        else { 
          imgd.width = image.width; 
          imgd.height = image.height; 
        } 
        imgd.alt = image.width + "x" + image.height; 
      } 
      else { 
        if (image.height > preh) { 
          imgd.height = preh; 
          imgd.width = (image.width * preh) / image.height; 
        } 
        else { 
          imgd.width = image.width; 
          imgd.height = image.height; 
        } 
        imgd.alt = image.width + "x" + image.height; 
      } 
    } 
  } 
//当idfile内容改变时 
  function filechange(value) { 
    flag = false; 
    document.getelementbyid("showimg").style.display = "none";    
    document.getelementbyid("idimg").width = 10; 
    document.getelementbyid("idimg").height = 10; 
    document.getelementbyid("idimg").alt = ""; 
    document.getelementbyid("idimg").src = value; 
  } 

<div class="cbs"> 
<div class="l"><label>图片:</label></div> 
<div> 
  <input id="idfile" name="pic" type="file" runat="server" onchange="filechange(this.value);" /> 
</div> 
    </div>  
    <div class="cbs"> 
<div class="l"><label>预览:</label></div> 
<div> 
  <img id="idimg" height="0" width="0" src="" alt="" onload="drawimage(this);" /> //实现预览 
  <img id="showimg" width="118" height="118" alt="" runat="server" style="display:none"/>  //加这个主要是为了实现查看时显示图片，因为上面的（idimg）加上runat="server"　报错，如有好的方法可以留言     
</div> 
</div>    

[ajax.ajaxmethod()] 
public bool savedata(string filenamepath) 
{ 
  string serverfilename = ""; 
  string sthumbfile = "";   
  string ssavepath = "~/files/"; 
  int intthumbwidth = 118; 
  int intthumbheight = 118; 
  string sthumbextension = "thumb_"; 
  try 
  { 
 //获取要保存的文件信息 
 fileinfo file = new fileinfo(filenamepath); 
 //获得文件扩展名 
 string filenameext = file.extension; 
 
 //验证合法的文件 
 if (checkfileext(filenameext)) 
 { 
   //生成将要保存的随机文件名 
   string filename = getfilename() + filenameext; 
   //检查保存的路径 是否有/结尾 
   if (ssavepath.endswith("/") == false) ssavepath = ssavepath + "/"; 
 
   //按日期归类保存 
   string datepath = datetime.now.tostring("yyyymm") + "/" + datetime.now.tostring("dd") + "/"; 
   if (true) 
   { 
 ssavepath += datepath; 
   } 
   //获得要保存的文件路径 
   serverfilename = ssavepath + filename; 
   //物理完整路径 
   string tofilefullpath = httpcontext.current.server.mappath(ssavepath); 
 
   //检查是否有该路径 没有就创建 
   if (!directory.exists(tofilefullpath)) 
   { 
 directory.createdirectory(tofilefullpath); 
   } 
 
   //将要保存的完整文件名  
   string tofile = tofilefullpath + filename; 
 
   ///创建webclient实例 
   webclient mywebclient = new webclient(); 
   //设定windows网络安全认证  
   mywebclient.credentials = credentialcache.defaultcredentials; 
   
   //要上传的文件 
   filestream fs = new filestream(filenamepath, filemode.open, fileaccess.read); 
   //filestream fs = openfile(); 
   binaryreader r = new binaryreader(fs); 
   //使用uploadfile方法可以用下面的格式 
   //mywebclient.uploadfile(tofile, "put",filenamepath); 
   byte[] postarray = r.readbytes((int)fs.length); 
   stream poststream = mywebclient.openwrite(tofile, "put"); 
   if (poststream.canwrite) 
   { 
 poststream.write(postarray, 0, postarray.length); 
   } 
   poststream.close(); 
   //以上为原图 
   try 
   { 
 //原图加载  
  using (system.drawing.image sourceimage = system.drawing.image.fromfile(system.web.httpcontext.current.server.mappath(serverfilename))) 
 { 
   //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  
  int file_append = 0; 
   sthumbfile = sthumbextension + system.io.path.getfilenamewithoutextension(filename) + filenameext; 
 
   while (system.io.file.exists(system.web.httpcontext.current.server.mappath(ssavepath + sthumbfile))) 
   { 
  file_append++; 
  sthumbfile = sthumbextension + system.io.path.getfilenamewithoutextension(filename) + 
file_append.tostring() + filenameext; 
   } 
   //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  
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); 
   return true; 
   } 
  } 
   } 
 } 
   } 
   catch 
   { 
 //出错则删除  
 system.io.file.delete(system.web.httpcontext.current.server.mappath(serverfilename)); 
 return false; 
   } 

 } 
 else 
 { 
   return false; 
 } 
  } 
  catch (exception e) 
  { 
 return false; 
  } 
} 
/// <summary> 
/// 检查是否为合法的上传文件 
/// </summary> 
/// <param name="_fileext"></param> 
/// <returns></returns> 
private bool checkfileext(string _fileext) 
{ 
  string[] allowext = new string[] { ".gif", ".jpg", ".jpeg" }; 
  for (int i = 0; i < allowext.length; i++) 
  { 
 if (allowext[i] == _fileext) { return true; } 
  } 
  return false; 
 
} 
   //生成随机数文件名 
public static string getfilename() 
{ 
  random rd = new random(); 
  stringbuilder serial = new stringbuilder(); 
  serial.append(datetime.now.tostring("yyyymmddhhmmssff")); 
  serial.append(rd.next(0, 999999).tostring()); 
  return serial.tostring(); 
 
}