Asynchronous PostBack during Image file upload

<asp:Panel ID="panelContent" runat="server" CssClass="modalPopup" Style="display:none";>
      <table>
               <tr>
                       <td colspan="3" align="left">
                              <asp:Label ID="Label15" runat="server" CssClass="RegLeftDtlGroupModal">Upload Your Profile Picture</asp:Label>
                        </td>
                </tr> 
                  <tr>
                        <td  colspan="3">
                           <hr />
                         </td>
                   </tr> 
                   <tr>
                        <td class="fstCol">
                        </td>
                        <td>
                         <table>
                                    <tr>
                                        <td class="LeftLabel">
                                            <asp:Label ID="Label16" runat="server" CssClass="RegLeftDtlLabel" AssociatedControlID="FileUploadThumb">Select an image file on your computer:</asp:Label>
                                        </td>
                                    </tr>
                                    <tr>
                                        <td class="LeftText">
                                            <asp:FileUpload ID="FileUploadThumb" ContentEditable="false" runat="server" CssClass="RegLeftDtlText" BackColor="#6699CC" BorderStyle="Groove" BorderColor="AliceBlue"/>
                                        </td>
                                    </tr> 
                                     <tr>
                                        <td class="LeftLabel">
                                            <asp:Label ID="Label1" runat="server" CssClass="RegLeftDtlLabel">Accepted files:.jpg,.gif,.png(4MB max)</asp:Label>
                                        </td>
                                    </tr>
                                    
                                    <tr>
                                        <td class="ErrMsg">
                                            <asp:RequiredFieldValidator ID="rfv1" runat="server" ValidationGroup="vPic" ControlToValidate="FileUploadThumb" Display="Dynamic" ErrorMessage="Image File Name should not be blank." ></asp:RequiredFieldValidator>
                                        </td>
                                    </tr>
                                </table>
                                
                           </td>
                           <td class="note"></td>
                           </tr>
                            <tr>
                                <td  colspan="3">
                                <hr />
                                </td>
                            </tr>          
                            <tr>
                                <td>
                                </td>
                                <td align="right">
                                    <asp:LinkButton id="btnUpdatePic" runat="server"
                                        CssClass="submit_button" Text="Update Image" onclick="btnUpdatePic_Click"></asp:LinkButton>
                                </td>
                                <td align="left">        
                                    <asp:LinkButton id="btnCancel" runat="server"
                                        CssClass="cancel_button" Text="CANCEL" ></asp:LinkButton>        
                                </td>
                            </tr>
                            <tr>
                                <td>
                                    <asp:Label ID="lblFileUploadError" runat="server" class="erLogin"></asp:Label>
                                </td>
                            </tr>
                        </table> 
                    </asp:Panel>
 
 
<ajax:ModalPopupExtender ID="ModalPopupExtender1" runat="server" 
             TargetControlID="panelExtender"
             PopupControlID="panelContent"
             BackgroundCssClass="modalBackground"
             DropShadow="false"
             OkControlID="btnUpdatePic"
             PopupDragHandleControlID="panelContent" Drag="true"
             CancelControlID="btnCancel" /> 
 
<asp:UpdatePanel ID="UpdatePanel4" runat="server">
                                                                 <ContentTemplate>
<div class="PlayerPic" style="background-image:url(Images/<%=PlayerProfileImage%>);" onmouseover="DivPlayerPicBoxOver();" onmouseout="DivPlayerPicBoxOut();" >
 
 
                                                              </ContentTemplate>
                                                                <Triggers>
                                                                    <asp:AsyncPostBackTrigger ControlID="btnUpdatePic" EventName="Click" />
                                                                </Triggers>
                                                            </asp:UpdatePanel>

                                  

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