By default, all RadEditor dialogs are embedded as resource fils in the
Telerik.Web.UI.dll file, which makes their customization harder. In order to
provide an easy way to modify the editor's dialogs, the RadControls installation
now comes with an extra folder named EditorDialogs
contains the dialogs' ascx files. You can configure RadEditor to use the
external dialog files by copying the EditorDialogs
the root of the web application and setting the
property. You can get more information on the matter in the following help article http://www.telerik.com/help/aspnet-ajax/customizingimagedialog.html
In addition you should know that in order to set the ExternalDialogsPath property you need to modify the respective ConfigFile.xml located in C:\Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\18.104.22.168__1f131a624888eeed\Resources folder.
I hope this helps.
All the best,
the Telerik team
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