Hello Birol,
For now, developing a WYSIWYG editor for Silverlight is not in our development plans.
The main idea behind RadRichTextBox is to load, edit and save rich text content. You can take a look of the help article about format providers and data providers (components used for data binding RadRichTextBox) - note that there are various image export options you can use if the default one doesn't fit your needs.
Don't hesitate to contact us if you have other questions.
Best wishes,
Boby
the Telerik team
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