XnView Software

A first way : copy the photo you choose and rename it as "folder.jpg" in each folder
Other method : rename this photo "1.jpg" to make it stay first in alphabetical order.

But it's difficult to refresh folders thumbnails...
The only strong method I found is to delete the entry of the parent ...

Is there any way to change what picture are shown as a folder's thumbnail ?

What I'm looking for is a setting similar to what can be done in Windows File explorer via:
Right click a folder->Properties->Customize Tab->Folder Pictures->Choose File

With the steps above, one can change the preview ...