I would like to change the thumbnail associated with a collection from the default (which I guess is the thumb of the most recent item uploaded to the collection?) to the thumb of the item listed first in the collection. So, if I change the order of the items (using the Item Order plugin), the collection will have a new thumbnail.
The representative file for collection is their latest and featured item image. If the collection doesn’t contain featured items, the most recently added item image will be used for representative image.
Reordering items within collection doesn’t change their date added so have no impact for sorting collection items by date added.
So you can’t do it by the Item Order plugin I guess.
The only way to make the collection to select a particular item image as its representation is to mark the item as featured.