AsyncFileUpload contorl doesn't work correctly ,,sometimes show image thumbnail and sometimes don't ,,PLZ HEEEELPPPP

JS and HTML Code :<script type="text/javascript">
        function uploadStarted() {$get("imgDisplay").style.display = "none";
        }
        function uploadComplete(sender, args) {
            var imgDisplay = $get("imgDisplay");
            imgDisplay.src = "images/loader.gif";
            imgDisplay.style.cssText = "";
            var img = new Image();
            img.onload = function () {
                imgDisplay.style.cssText = "height:100px;width:100px";
                imgDisplay.src = img.src;
            };
            img.src = "<%=ResolveUrl(UploadFolderPath) %>" + args.get_fileName();
        }</script><div class="fileupload fileupload-new" data-provides="fileupload"><div class="fileupload-new thumbnail" style="width: 200px; height: 150px;"><img id="img1" src="" runat="server" alt="" /></div><div class="fileupload-preview fileupload-exists thumbnail" style="max-width: 200px; max-height: 150px; line-height: 20px;"></div></div><%--<asp:FileUpload ID="fuImg" runat="server" />--%><asp:UpdatePanel ID="UpdatePanel1" runat="server"><ContentTemplate><cc1:AsyncFileUpload OnClientUploadComplete="uploadComplete" runat="server" ID="AsyncFileUpload1"
                                        Width="400px" CompleteBackColor="White" UploadingBackColor="#CCFFFF"
                                        ThrobberID="imgLoader" OnUploadedComplete="FileUploadComplete"  OnClientUploadStarted="uploadStarted" /><asp:Image ID="imgLoader" runat="server" ImageUrl="images/loader.gif" /><br /><br /><img id="imgDisplay" alt="" src="" style="display: none" /></ContentTemplate></asp:UpdatePanel></div></div>

//C# Code:
protected string UploadFolderPath = "~/admin/Uploads/";


protected void FileUploadComplete(object sender, EventArgs e)
    {
        string extension = Path.GetExtension(AsyncFileUpload1.PostedFile.FileName);
        string filename = System.IO.Path.GetFileName(AsyncFileUpload1.FileName + "-" + DateTime.Now.Millisecond) + extension;
        AsyncFileUpload1.SaveAs(Server.MapPath(this.UploadFolderPath) + filename);
        img1.Src = "~/admin/Uploads/" + filename;
        Session["ImageSaved"] = filename;
    }