Hello Jörg B.,
RadRichTextBox supports only inline images for now. We plan to extend this functionality, but I cannot provide you with a specific timeline of when this functionality will be implemented. We have a PITS issue about this feature at http://www.telerik.com/support/pits.aspx#/public/silverlight/4944, which you can use to track our progress.
All the best,
the Telerik team
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