It appears to be completely random to some users' consternation. Sometimes manipulating the contents of the note will change the displayed thumbnail, sometimes it won't. I agree that there needs to be some sort of predictable method for which image is displayed. I've got hundreds of images in my historical research and some of the random displayed images are completely useless (like the backs of buildings, zoomed-in pieces of architecture, or fragments of paintings and statues. I'm hoping that if the card view is here to stay that its functionality will at least be improved for more practical uses.