XnView Software

Right now it is not possible to determine witch pic inside a folder is used as thumbnail or preview for this folder. It seems to be the first one. But if you change names and get another pic as first one, the preview stays the same.

xnview wrote: It's the first one found by the system not name sorted. Perhaps it will be good to have the first one alphabetically

I suggest making this change. It would solve the problem. The user could put a zero ahead of the name of the pic he wants to have as preview or make a copy named 000 or something like that.

Just take any image you want displayed as folder thumbnail and name it folder.jpg.. that is the simplest thing to do I think.
I use screen cap software, select region of image and have it scale down to certain resolution and names it folder.jpg