Please, see the following help article - http://www.telerik.com/help/radeditormoss/SetPropertiesViaConfigFile.html
. It describes how to change the editor configuration and shows how to add the SiteCollectionImages library in the ImageManager. You can use the same approach to use the library in the FlashManager tool.
All editor dialogs check the user access using the SharePoint API. This means that if a user has rights to add list items, they will be able to upload content using the editor dialogs.
the Telerik team