Adding ImageEditor inside a RadPanel breaks the RadPanels designer. The problematic behavior can be observed with the following configuration:

        <telerik:RadPanelBar ID="RadPanelBar1" Runat="server">
            <Items>
                <telerik:RadPanelItem runat="server" Text="Root RadPanelItem1">
                    <ContentTemplate>
                        <telerik:RadImageEditor ID="RadImageEditor1" runat="server" ImageUrl="~/Images/Jellyfish - Copy.jpg"></telerik:RadImageEditor>
                        <asp:Button runat="server" ID="dasd" />
                        <telerik:RadEditor ID="RadEditor1" runat="server"></telerik:RadEditor>
                    </ContentTemplate>
                </telerik:RadPanelItem>
            </Items>
        </telerik:RadPanelBar>