Office 365 Delve: Which picture does Delve use for the thumbnail?

I have been using Delve for a while now, and one thing has struck me as odd, and that is the image used as the thumbnail and how it was being chosen.  As an example I was writing a document in Word and saved it to my OneDrive.

Initially it displayed one image, however when I edited the document and added an additional image, the view on Delve changed.

I wasn’t sure why this was the case and it is something I had seen in particular with PowerPoint decks, and to me it did seem a bit random.  Apparently the algorithm used picks the image with the highest resolution within the document.  So there you go, not random at all and has a certain logic behind it.