When you set Lightweight render mode via web.config, the image editor appears broken.

You can use the following setup in web.config as a workaround for the time being. Note that all instances of the RadToolBar will be rendered in Classic mode.
 <appSettings>
      ....
      <add key="Telerik.Web.UI.RenderMode" value="Auto" />   
      <add key="Telerik.Web.UI.ToolBar.RenderMode" value="Classic" />
       ....
</appSettings>