I'd suggest you to investigate plugin's css sources - you can find it here:
<SVN root folder>\Ext.Net\Build\Ext.Net\ux\plugins\tabmenu\resources\css\tabmenu.css
The css rule to override that icon looks like this:
.x-tab-strip .x-tab-strip-withmenu a.x-tab-strip-menu {
background-image: url(resources/images/my-tab-menu.gif);
}
Example
<%@ Page Language="C#" %>
<%@ Register Assembly="Ext.Net" Namespace="Ext.Net" TagPrefix="ext" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title>Ext.NET Example</title>
<ext:ResourcePlaceHolder runat="server" Mode="Style" />
<style type="text/css">
.x-tab-strip .x-tab-strip-withmenu a.x-tab-strip-menu {
background-image: url(resources/images/my-tab-menu.gif);
}
</style>
</head>
<body>
<form runat="server">
<ext:ResourceManager runat="server" />
<ext:TabPanel runat="server">
<Items>
<ext:Panel runat="server" Title="Tab 1">
<TabMenu>
<ext:Menu runat="server">
<Items>
<ext:MenuItem runat="server" Text="Hello!" />
</Items>
</ext:Menu>
</TabMenu>
</ext:Panel>
<ext:Panel runat="server" Title="Tab 2">
<TabMenu>
<ext:Menu runat="server">
<Items>
<ext:MenuItem runat="server" Text="Hello!" />
</Items>
</ext:Menu>
</TabMenu>
</ext:Panel>
</Items>
</ext:TabPanel>
</form>
</body>
</html>