Page Previews rejects some small images which have been selected as page image: Extension:Popups#Known problems. Topic:X24ym9nooumpgr1h tries to state the requirement more clearly. There is an example here: How come some images show up while hovering on some articles and on others they do not? Popups is a different extension but it's a prominent use of PageImages and can confuse users so I suggest a brief mention here, maybe my opening sentence.
