Interesting. I've not come across the 'transformative' argument before and I wonder if that is a US thing, because UK definitions I've seen don't seem to include 'transformative' works. Prince hasn't actually changed some of the images, and he's barely changed them where he has. Putting a blob over someone's face, or including the Instagram frame doesn't makes it his work. He should buy a licence, in my opinion. They're often cheap!