判断上传图片的脚本

<script>
var loadNum=0;
var loadTimes = 3;
var checkShowSize = false;
var alertmsg = "您选中的图片有问题，请换张图片上传";
function PriviewPic()
{     
    if(document.Form1.TextFieldUpPic.value=="")
    {
    return true;
    }  
 document.all.view.src = document.Form1.TextFieldUpPic.value;
 var width=500; 
    var maxfilesize = 204800;
    var showimg = new Image();
    showimg.src = document.Form1.TextFieldUpPic.value;  
    if(showimg.width > width)
 {
   var ori_w= showimg.width;
   var ori_h = showimg.height;
   showimg.width = width;
   showimg.height = ori_h * showimg.width / ori_w;
 }    
 document.all.view.style.width = showimg.width;
 document.all.view.style.height = showimg.height;
    if(showimg.fileSize>0)
 {
  if(showimg.fileSize > maxfilesize)
  {
   alertmsg = "您选中的图片大于" + maxfilesize+ ",目前字节数=" + showimg.fileSize;
   return false;
  } 
  checkShowSize = true;
 }
    else if(loadNum<loadTimes)
         {
       loadNum ++ ;
       setTimeout('PriviewPic()',500); 
      }
 else if(loadNum>=loadTimes)
      {
       loadNum = 0;
       alertmsg = "您选中的图片有问题，请换张图片上传";
       return false;
      } 
}
function UpPic()
{  
    if(document.Form1.TextFieldUpPic.value=="")
    {
        return true;
    }
 if(!checkShowSize)
 {
  setTimeout('PriviewPic()',500);
  alert(alertmsg);
  return false;
 }
 var filename = document.Form1.TextFieldUpPic.value.toLowerCase();
 var filext = filename.substring(filename.length-3,filename.length);
 if(filext!="jpg" &&  filext!="gif")
 {
  alert("您上传的图片不是jpg或gif格式");
  return false;
 } 
 return true; 
}
</script>