Henrik's right about the Silverlight clipboard and the options it provides. That is why it would not be possible to obtain anything more than plain text from the clipboard and paste it in the rich text box.
When it comes to the InsertImage method of RadRichTextBox, it takes a stream and an extension, which specifies what is the format of the image and inserts the image at the current caret position. For example, this method is used when the InsertPictureCommand is triggered. The same method is used in our online demo when you drop an image file on the document:
sender, DragEventArgs e)
FileInfo droppedFiles = e.Data.GetData(DataFormats.FileDrop)
(Stream imageStream = droppedFile.OpenRead())
Therefore, as long as you have the image stream and know its format, you can insert it in the document.
the Telerik team
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