That said, a simpler way to handle this would be that the image generator model is a tool. And it doesn’t really matter if your input is through a paintbrush or a prompt in an image generator, you’re responsible for that piece of content thus you have copyright over it.
You can further couple it with the argument @luciole used, that AI art is derivative work, and claim that the authors of the works used to train the model shall have partial copyright over it too.