A workaround is to set the icon URL from the server in order to resolve the URL yourself. For example:

<telerik:RadNotification ID="rnNotInstalledLicense" runat="server" AutoCloseDelay="0" 
						 ShowCloseButton="true" Height="130px" Width="350px" VisibleTitlebar="true"
						 ContentIcon="~/images/install_dataagent.gif"
						 EnableEmbeddedSkins="false" />
<asp:Button ID="Button1" Text="show notification" OnClientClick="showNotification();

return false;" runat="server" />
<script>
	function showNotification() {
		$find("<%=rnNotInstalledLicense.ClientID%>").show();
	}
</script>

and on the server:

C#:
protected void Page_Load(object sender, EventArgs e)
{
	if(!IsPostBack)
	{
		rnNotInstalledLicense.ContentIcon = Page.ResolveUrl(rnNotInstalledLicense.ContentIcon);
	}
}


VB:
Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load
	If Not Page.IsPostBack Then
		rnNotInstalledLicense.ContentIcon = Page.ResolveUrl(rnNotInstalledLicense.ContentIcon)
	End If
End Sub