<telerik:RadAjaxPanel ID="Loadingpanel1"  runat="server">
    <asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
<telerik:RadAjaxManager ID="RadAjaxManager1" runat="server" DefaultLoadingPanelID="RadAjaxLoadingPanel1">
           <ClientEvents  OnRequestStart="RequestStart()" />
            <AjaxSettings>
                <telerik:AjaxSetting AjaxControlID="Loadingpanel1">
                    <UpdatedControls>
                        <telerik:AjaxUpdatedControl ControlID="Loadingpanel1" LoadingPanelID="RadAjaxLoadingPanel1" />
                    </UpdatedControls>
                </telerik:AjaxSetting>
            </AjaxSettings> 
        </telerik:RadAjaxManager>

 <telerik:RadAjaxLoadingPanel ID="RadAjaxLoadingPanel1" runat="server"  Transparency="30"  IsSticky="true" CssClass="MyModalPanel" Skin="" ScrollBars="None">
                 <asp:Image ID="Image1"  runat="server" AlternateText="Wird geladen" Style="" ImageUrl=  "~/App_Themes/WillisDRITheme/Images/Processing.gif"  CssClass="MyLoadingImage"/>
        </telerik:RadAjaxLoadingPanel>   <asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
                </asp:ContentPlaceHolder>
</telerik:RadAjaxPanel>

function RequestStart() {

// the following Javascript code takes care of expanding the RadAjaxLoadingPanel

// to the full height of the page, if it is more than the browser window viewport

var loadingPanel = document.getElementById("<%= RadAjaxLoadingPanel1.ClientID %>");

var pageHeight = document.documentElement.scrollHeight;

var viewportHeight = document.documentElement.clientHeight;

if (pageHeight > viewportHeight) {

loadingPanel.style.height = pageHeight + "px";

}

// the following Javascript code takes care of centering the RadAjaxLoadingPanel

// background image, taking into consideration the scroll offset of the page content

var scrollTopOffset = document.documentElement.scrollTop;

var loadingImageHeight = 55;

loadingPanel.style.backgroundPosition = "center " + (parseInt(scrollTopOffset) + parseInt(viewportHeight / 2) - parseInt(loadingImageHeight / 2)) + "px";

// workaround for RadAjaxLoadingPanel for ASP.NET - there are two elements with the same ID

// this is not needed with RadAjaxLoadingPanel for ASP.NET AJAX

if (loadingPanel.nextSibling.className == loadingPanel.className) // IE, Opera

{

loadingPanel.nextSibling.style.backgroundPosition = "center " + (parseInt(scrollTopOffset) + parseInt(viewportHeight / 2) - parseInt(loadingImageHeight / 2)) + "px";

}

else if (document.getElementsByClassName) // Firefox

{

var panels = document.getElementsByClassName("MyModalPanel");

for (var j = 0; j < panels.length; j++) {

panels[j].style.backgroundPosition = "center " + (parseInt(scrollTopOffset) + parseInt(viewportHeight / 2) - parseInt(loadingImageHeight / 2)) + "px";

}

}

}

.MyLoadingImage

{

position:relative;

top:50%;

margin-top:-30px;

left:50%;

margin-left:-65px;

}