It should still be like before, you just click on the "double building" icon, and directly beneath it there should be 3 small spaces with small emblems in them. Its one the three.
So, Ive been putting custom thumbnails on my workshop creations for some months now, but since the Advanced Building Update, the Thumbnails do not show up in the "Creations" Tab ingame.
They still are inside the folder and taking them out and putting them back in doesn't fix it.
Im kinda out of...