By design, when in Flow LayoutMode, the content of the document is flowing vertically and document width is always equal to available width in the RichTextBox. Thus no horizontal scrollbars are meant to be shown.
You can see that in MS Word when inserting a big picture in web Layout mode, a horizontal scroll-bar is shown, but when attempting to print the document for example, the picture is still being cropped.
In MS RichTextBox, the picture is being resized along with the rich textbox.
Overall, there are different approaches that can be adopted. We will consider them, but cannot promise anything specific.
the Telerik team
Browse the videos here>>
to help you get started with RadControls for Silverlight