By the current implementation of the Phereo website, it seems the last uploaded image is to be set as the album thumbnail.
When the last image visibility is set in the limited, it seems to be shown as "EMPTY ALBUM", even though it contains other images (OK, better than that limited image is to show up as it used to be).
It would be nice to allow a user to pick up a preferrable image as the album thumbnail.
(Somehow, my albums created from the 10th to the 11th do not have their album thumbnails though they contained only public images...
Although it looks already OK for a new album created today, I have no way to recover them...
The subjected solution is to help such a problem as well, I am afraid.)