sub Image_data::location {
        my $O = shift;
        
        my $location = $O->local_storage() || $O->default_storage();
        return ($location);
    }
    
    sub Image_data::local_storage {
    =pod
        returns the path to a file on HD if the image is stored
        locally, and a null string if the image is not stored 
        locally.
    =cut
    }
    
    sub Image_data::default_storage {
    =pod
        combines ID info from the parent Image and CD objects
        to create a filepath for the image on CD.   this routine
        must *always* return a valid filepath.
    =cut
    }