hi
check out the following
<asp:ModalPopupExtender ID="pe_pictureDetails" runat="server" Y="10" RepositionMode="None" BehaviorID="pe_pictureBehaviour"
DynamicServicePath="" Enabled="True" TargetControlID="btn_picturePopup"
PopupControlID="pnl_Picture" PopupDragHandleControlID="pnl_PictureHeader"
BackgroundCssClass="modalBackground" DropShadow="false" EnableViewState="true" >
<Animations>
<OnShown>
<FadeIn duration="0.15" Fps="100" />
</OnShown>
</Animations>
</asp:ModalPopupExtender>