This is available in version 1.4 and older. The page thumbnails are not configurable.

To display a page thumbnail:

<l:Image 

         HorizontalAlignment="Center" 

         Width="80" 

         VerticalAlignment="Center" 

         Stretch="Uniform" 

         ImageId="{Binding Path=[page.thumbnail.id]}" 

/>

See also

Displaying page thumbnails (default option - v1.5 or newer)

Displaying page thumbnails (overriding - v1.5 or newer)