Asp. Net upload image verification code small example

<script language="javascript">
img=new Image() ;
function Check_FileTypeAndFileSize()
{
    // Get the upload path 
    var str=document.all.uploadFile.value;
    // Verify that the upload path is not empty 
    if(str=="")
    {
          alert(" Please select the image to upload first! ");
          return false;
    }
    // Verify that the uploaded file format is correct 
    var pos = str.lastIndexOf(".");
    var lastname = str.substring(pos,str.length) 
    if (lastname.toLowerCase()!=".jpg" && lastname.toLowerCase()!=".gif")
    {
         alert(" The file type you uploaded is " lastname " , the picture must be  JPG  , GIF  type ");
         return false;
    }
    // Verify the uploaded file width/height ratio 
    if(img.width/img.height>1.6)
    {
         return confirm( The proportion of pictures you upload is greater than 8:5 Are you sure you want to upload? );
    }
    // Verify that the uploaded file is out of size 
    if(img.fileSize/1024>150)
    {
         alert(" The file size you uploaded is out of tolerance 150K Limit! ");
         return false;
    }
    return true;
}
// Upload file box address change event, instant preview image 
function changephoto()
{
img.src=document.all.uploadFile.value;
}
//-->
</script>
<div>
                <Upload:InputFile ID="uploadFile" name="UploadFilePath" runat="server" onchange="changephoto() " />
                <asp:Button ID="btnUpload" runat="server" Text=" upload " OnClientClick="return Check_FileTypeAndFileSize()"
OnClick="btnUpload_Click" />
</div>
<div class="upload">
                 <img id="imgPreview" src="images/pic1.gif" runat="server" />
</div>