Thanks for the heads up! I didn't know that. It's a maybe unfortunate side effect of the MIT license, but it does allow people to do whatever they want with the planet images, including selling them.
I don't really care about the credit but it does suck for the people who pay for something when they can get that same thing for free. So not too much I can do about it, but thanks for letting me now! Perhaps you can still request a refund from itch if you want?