Why doesn't ajaxModalExtender hide ?

i have used AjaxModalExtender, it works but problem is that when it appears then it

<asp:image ID="ImageLetterCopy"  style="max-width: 100%;max-height: 100%"   runat="server" />

doesn't hide anyway. I want to hide it, how can I do it ? 

<asp:HiddenField ID="hdnField1" runat="server" /><asp:ModalPopupExtender ID="mdlMessageBox" BackgroundCssClass="modalBackground" runat="server" TargetControlID="hdnField1"
                    PopupControlID="ImageLetterCopy"></asp:ModalPopupExtender>

i just want to hide it when some one clicks anywhere on page or any other simple way.