CheckBox inside ASP.Net GridView does not fire CheckedChanged event when unchecked

I am using the Nested Grid example, but i have a courious problem:

I have some checkboxes inside the Nested Grid and when i put the image (plus/minus) inside the ItemTemplate tag, the OnCheckedChanged event of the Checkbox does not fire.

The OnCheckedChanged only fires when I check the box. The unchecked do not fire the event. 

I'm also tried to change the MasterPage to use EnableViewState=true and the page itself to. Nothing. I've changed the checkboxes with ViewStateMode=Enabled and EnableViewState=true. Still nothing. 

If I remove the image tag, everything works (with or without the changes I've made), but the Nested effect don't appear. 

Remove the image, everything works ok.

Any ideias?

Thank you in advance!

                       <asp:TemplateField HeaderText="Representantes">
                            <ItemTemplate>
                                <img alt = "" style="cursor: pointer" src="images/plus.png" />  
                                <asp:Panel ID="pnlRepr1" runat="server" Style="display: none">
                                    <asp:Label ID="lblRper2" runat="server" Width="245px" Text="Representantes" />
                                    <asp:GridView ID="gdvRepr" runat="server" AutoGenerateColumns="False" CssClass="GridAlternating" CellPadding="2"
                                    GridLines="none" ShowHeader="true" Width="100%" BorderStyle="None" OnRowCommand="gdvRepr_RowCommand" EnableViewState="true">
                                    <Columns> 
                                    <asp:TemplateField HeaderText="Assinar" >                                        
                                        <ItemTemplate>
                                            <asp:CheckBox ID="chkPublicar" runat="server" Checked='<%# Bind("repr_fl_publicar") %>' AutoPostBack="true" OnCheckedChanged="associadoAtualizar" />                                                
                                        </ItemTemplate>
                                         <ItemStyle Width="30px" HorizontalAlign="Center" />
                                    </asp:TemplateField>
                                    <asp:TemplateField HeaderText="Acompanhar" >
                                        <ItemTemplate>
                                            <asp:CheckBox ID="chkAcompanhar" runat="server" Checked='<%# Bind("repr_fl_acompanhar") %>' AutoPostBack="true" OnCheckedChanged="associadoAtualizar" />
                                        </ItemTemplate>
                                        <ItemStyle Width="28px" HorizontalAlign="Center" />
                                    </asp:TemplateField>                                    
                                    <asp:TemplateField HeaderText="Aprovar" >
                                        <ItemTemplate>
                                             <asp:CheckBox ID="chkAprovar" runat="server" Checked='<%# Bind("repr_fl_aprovar") %>' AutoPostBack="true" OnCheckedChanged="associadoAtualizar" />
                                        </ItemTemplate>
                                        <ItemStyle Width="28px" HorizontalAlign="Center" />
                                    </asp:TemplateField>                                        
                                    </Columns>
                                    <RowStyle VerticalAlign="Top" />
                                </asp:GridView>
                                </asp:Panel>
                                </ItemTemplate>
                            </asp:TemplateField>