You should be able to control whether or not a user can paste in an image into the RadEditor by editing the OnClientPasteHtml event. You will want to check for the Paste event execution and for the used browser ($telerik.isFirefox), then strip the img tags using regular expression or DOM method from the content returned by the args.get_value() method. After that set the new content via the args.set_value(content) method.