Even if it's the paid version, the prompter had it make the infringing content for their own use. And it's only going to be anything more than equivalent to someone making their own sketch of Batman if the prompter reproduces and distributes. It's not like it's illegal for you to tell your artist friend to sketch a Batman either - Comic Cons and such are full of that.
So you need a legal process to determine if that generation was actually infringing on copyright.
IANAL so I have no idea really, and I think most of the other commenters here can’t say for sure either.
Prompters aren't artists, and neither are image generators. The corporations that create these generators are responsible for the content they illicitly fed into them that they are now spewing out.
If you sell me an endless coloring book that occasionally spits out copyrighted content, that's on you.