This would be a desaster for me. The selected thumbnail for the folder is a once very well choosen picture. Changing it randomly, again, a desaster!

I have also hundrets of videos, which „thumb“ should be used if randomly a video delivers the thumb?

No, thanks, I don't see any sense in such a function for me. But, don't get me wrong, this might be interesting for others.

What skin are you using? A skin could pick a random image for you using javascript for instance.

Otherwise, you can randomise images in jAlbum by right-clicking a folder and selecting Order by->Shuffle.

To randomize all representing images for all sub folders, run this script in jAlbum's system console (F7). It's platform neutral as well, and won't break if we update jAlbum's underlying files: (Groovy mode)

import java.util.concurrent.ThreadLocalRandom;
rootFolder.descendants.filter(ao -> ao.folder).forEach(folder -> {
  folder.representingAlbumObject = folder.children[ThreadLocalRandom.current().nextInt(folder.children.size())]
})

To save this script, save it as "Randomize representing images.groovy" to <config>/tools/, where "<config>" is jAlbum's config folder (Tools->Open directories->Config directory).
After a restart, you can now access this tool via jAlbum's Tools menu at any time.