How to disable Jquery DateTimePicker image button.

function disableDate() {
    debugger;
    document.getElementById('txtStartDate').disabled = true;
    $get('txtStartDate').style.color = "gray";
    $get('txtStartDate').disabled = true;




    $('txtStartDate').datepicker('disable');

    $('txtStartDate').datepicker('setDate', new Date()).datepicker('disable').blur();


    $('txtStartDate').disableSelection = true;


}

<input id="txtStartDate"  runat="server" class="TextBox DatePicker" 
                                            style="width: 125px" />

        <style>
 .ui-datepicker-trigger
        {
            position: relative;
            top: 4px;
            right: -3px;
            height: 17px;
        }
        </style>
        <script language="javascript" type="text/javascript">
            var Dformat = "";
            $(document).ready(function () {
                var selectvalStartDate = $("#txtStartDate").val();
                var sVal = document.getElementById('hdnPORTALID').value;
                Dformat = GetDatePickerFormat(sVal);
                $(".DatePicker").attr("placeholder", "Select date").datepicker({
                    showOn: "both",
                    buttonImage: "../images/datepicker_enable.png",
                    buttonImageOnly: true,
                    changeMonth: true,
                    changeYear: true,
                    dateFormat: Dformat
                });

            });
</script>