If a human being gets asked to draw the joker, gets a still from the film, then copies it to the best of their ability. They can’t sell that image. Technically speaking they’ve broken the law already by making a copy.
Is this really true? Breaking the law implies contravening some legislation which in the case of simply drawing a copyrighted character, you wouldn't be in most jurisdictions. It's a civil issue in that if some company has the rights to a character and some artist starts selling images of that character then whoever owns the rights might sue that artist for loss of income or unauthorised use of their intellectual property.
Regardless, all human artists have learned from images of characters which are the intellectual property of some company.
If I hired a human as an employee, and asked them to draw me a picture of the joker from some movie, there's no contravention of any law I'm aware of, and the rights holder wouldn't have much of a claim against me.
As a layperson, who hasn't put much thought into this, the outcome of a claim against these image generators is unclear. IMO, it will come down to whether or not a model's abilities are significantly derived from a specific category of works.
For example, if a model learned to draw super heros exclusively from watching marvel movies then that's probably a copyright infringement. OTOH if it learned to draw super heroes from a wide variety of published works then IMO it's much more difficult to make a case that the model is undermining the right's holder's revenue.