Please see the snapshot - I just updated my Telerik version from 2012 Q1 SP to 2012 Q2 and I'm unable to see the icons anymore. It was working fine before the update - I have following stylesheets added too:
<telerik:RadCodeBlock ID="rcbPage" runat="server">
<link href='<%= Telerik.Web.SkinRegistrar.GetWebResourceUrl(this, typeof(RadEditor), "Telerik.Web.UI.Skins.Editor.css") %>' rel="stylesheet" type="text/css" />
<link href='<%= Telerik.Web.SkinRegistrar.GetWebResourceUrl(this, typeof(RadEditor), "Telerik.Web.UI.Skins.Default.Editor.Default.css") %>' rel="stylesheet" type="text/css" />
<link href='<%= Telerik.Web.SkinRegistrar.GetWebResourceUrl(this, typeof(RadWindow), "Telerik.Web.UI.Skins.Window.css") %>' rel="stylesheet" type="text/css" />
<link href='<%= Telerik.Web.SkinRegistrar.GetWebResourceUrl(this, typeof(RadWindow), "Telerik.Web.UI.Skins.Default.Window.Default.css") %>' rel="stylesheet" type="text/css" />
</telerik:RadCodeBlock>
<telerik:RadEditor runat="server" ID="RE" EditModes="Design" ToolbarMode="Default" NewLineMode="Br"
ToolsFile="~/controls/BasicTools.xml" DialogsCssFile="~/skins/A/Dialog.A.css">
</telerik:RadEditor>