Storing in XDG directories is ticketed upstream at http://trac.yorba.org/ticket/421 .
As noted there, Shotwell isn't yet smart enough to automatically regenerate missing thumbnails (that's http://trac.yorba.org/ticket/2889), and we can't store thumbnails in a cache directory until that functionality is implemented.
Storing in XDG directories is ticketed upstream at http:// trac.yorba. org/ticket/ 421 .
As noted there, Shotwell isn't yet smart enough to automatically regenerate missing thumbnails (that's http:// trac.yorba. org/ticket/ 2889), and we can't store thumbnails in a cache directory until that functionality is implemented.