tab not focus/select datepicker image

I have a form with multiple textboxes and datepicker. When I do tab, the cursor goes from one text box to another textbox but the datepicker image is not selected with tab.

I've to use mouse to select the datepicker. How can I solve this problem ?

<div class="form-group"><label>Date Of Birth</label><asp:TextBox ID="txtDOB" runat="server" CssClass="txt-control" /><asp:Image ID="calendarImg1" runat="server" ImageUrl="~/images/calendar_month.gif"
                                        Style="cursor: pointer" /><ajaxToolkit:CalendarExtender ID="CAL" runat="server" Format="yyyy-MM-dd" PopupButtonID="calendarImg1"
                                        TargetControlID="colDateFrom" CssClass="calender" /></div>