Ajax control panel icon image not working from c# code-behind

[rhj4]

I am using an AJAX Control panel and setting the Expand/Collapse icon images from C#, but the icon images are not being found. This code works fine in Chrome and FireFox, but it doesn't seem to work in Opera.

C# code:

        cpGeneral.Collapsed = true;
        cpGeneral.ClientState = "true";
        cpGeneral.ExpandedImage = URLExpand;
        cpGeneral.CollapsedImage = URLCollapse;

where the URL is a constant. The declaration of the Control panel is:

<asp:CollapsiblePanelExtender ID="cpGeneral" runat="server" TargetControlID="pnlGeneral"
BehaviorID="cpGeneral" TextLabelID="lblGeneral" SuppressPostBack="true" Collapsed="true"
ImageControlID="icnGeneral" ExpandControlID="pnlGeneralcp" CollapseControlID="pnlGeneralcp"
ExpandedText="Collapse" CollapsedText="Position/Organization">
/asp:CollapsiblePanelExtender
<asp:Panel ID="pnlGeneralcp" runat="server">
<asp:ImageButton ID="icnGeneral" runat="server" CssClass="cpIcon" OnClientClick=" checkPanel('General');return false;" />
<asp:Label ID="lblGeneral" runat="server" Text="Position/Organization " />
/asp:Panel
<%-- GENERAL PANEL DATA --%>
<asp:Panel ID="pnlGeneral" CssClass="flsaInputPanel" runat="server">
...