It probably defaults to the database because that is the only location known to exist at install time. The Tiki installer cannot guarantee that users will have already created a "MyBigFatFolderToStoreAllMyTikiImages" folder. (extreme example but you get the general idea). The database is the only consistent location that will be true for all users regardless of platform, software etc etc. and consequently should work for all default installs without further action needed by the user.