If someone is selling art from my game? I definitely want to know about that. If someone is breaking the rules I explicitly defined in the description? I want to know that too.
But if someone just wants to pretend they drew an image from the game? And they're not making any kind of profit? Please, just do what I do: roll your eyes and move on. Seriously, check the description. I don't actually say people have to give me credit when they upload images, because that would be impossible to police and it's not worth anyone's effort.
To reiterate: If I specifically disallow something in the description (points adoptables, for-profit images, hosting the game elsewhere, etc.), then feel free to point that out to the person in question or to bring it to my attention. If I don't specifically disallow something in the description, just assume that's because it wasn't worth my time, and consequently isn't worth yours.