javascript:

                    function Loadimage(sender, argument) {
                        var strimage = argument;
                        var _strimage;
                        _strimage = strimage.replace(/["]/g, '');
                        _strimage = 'url(' + strimage.replace(/["']?\)$/, '') + ')';
                        var divimage = $get("<%= imgprofile100.ClientID %>");
                        divimage.style.backgroundImage = _strimage;
                    }

codice aspx:
                        <telerik:RadWindow ID="RadWindowLoadprofile" runat="server" ShowContentDuringLoad="false" Width="452px" Height="430px"
                            Title="Caricamento immagine profilo" Behaviors="None" Modal="True" Behavior="None" Font-Names="Verdana" Font-Size="Small" IconUrl="/image/iconlogo.png" 
                            Skin="Simple" ReloadOnShow="True" VisibleStatusbar="False" RestrictionZoneID="NavigateUrlZone" NavigateUrl="UploadImgProfile.aspx" OnClientClose="Loadimage">                           
                        </telerik:RadWindow>