Hi Louster1950
Thanks a lot for your contribution.
Actually I already have as a standard preference "by arrangement" on at the menu bar on the top, like you mentionned.
In fact, my problem is not how to arrange thumbnails, but how to keep the arrangement saved (if this is possible), so that when I open again the contact sheet an hour or a day later, I get the same arrangement as when I closed it before (of course, as long at the file hasnt itself changed).
Anyway, thanks again.
Noukita