I have read that paid images do better on Pinterest. I don’t think this has to do with the image so much as it does that the image is more unique. When I was using free images, I would often find a ton of other pins using the same image for a wide-range of topics. I believe this confuses the Pinterest search algorithm. This can still happen with paid images but its less likely. And as mentioned above, it’s not that much money for the peace of mind that you won’t later be notified that the image was not royalty free.