ftp is a different mechanism for uploading files, it isn't really related to site administrators because ftp is not governed by mojoPortal roles, so it is based on the user account for ftp and not at all related to mojoPortal users.
When editing content and adding images, the editor provides a way for users to browse and upload files on the server, it is not usually done by ftp, ftp is mainly for when you are upgrading your installation of mojoPortal or if you find it easier to upload lots of images by ftp some people may do that.
You cannot use the mojoPortal file manager for upgrading but you can use it for uploading files if you want to.
Hope that helps,
Joe