I dont remember every feature of old old versions but category or album thumbnails do not update to the latest. Only the latest would show if there is no thumbnail set. If you have a catavatar as it is called set nothing is ever updated. I do not ever remember a category or album thumbnail always being the latest image. When the ability to set an album thumbnail was introduced it was a manual thing which an admin could tag a photo in a category or album to be the avatar. It pretty much has not changed since its inclusion. I do know the one change made a few versions ago was that if an image is uploaded to a category or album that is empty meaning there is no thumbnail set for the category the code will update the avatar to that image. That of course will only happen once in a category or album otherwise you tag the thumbnail.