If you are using the built-in RadEditor dialogs, then it is possible to browse document libraries only from the current site and its parent sites. By default, the editor uses only the document libraries from the current site. If you want to use a library from a parent site, you need to set a property in the editor's configuration file. See the following article for more details: http://www.telerik.com/help/radeditormoss/SetPropertiesViaConfigFile.html
The above link shows how to add the Site Collection Images library in the editor's image browser.
the Telerik team