<script>
     function OnClientLoad(sender, args) {
         //disable drag and drop of external elements and text in Design mode
         $telerik.$(sender.get_contentArea()).bind("drop", function (e) {
             $telerik.cancelRawEvent(e);
         });
         //disable drag and drop from external elements and text in HTML mode
         $telerik.$(sender.get_textArea()).bind("drop", function (e) {
             $telerik.cancelRawEvent(e);
         });

         //disable drag and drop of elements and text inside RadEditor content area
         var element = document.all ? editor.get_document().body : editor.get_document();
         var eventHandler = document.all ? "drop" : "dragstart";
         $telerik.addExternalHandler(element, eventHandler, function (e) {
             $telerik.cancelRawEvent(e);
         });
     }
 </script>
 <telerik:RadEditor OnClientLoad="OnClientLoad" ID="RadEditor1" runat="server">
     <Content>
 Try to drag and drop the <img src="Images/Apply.png" /> image in the end of this line.</div>
     </Content>
 </telerik:RadEditor>
 <br />
 <div style="background-color: yellow; display: inline;">Select and drag the image
     <img src="Images/Apply.png" />
     into the editor's content area</div>
 <div style="background-color: yellow; display: inline;">Select and drag the text into the editor's content area</div>