Well, and both the AI and our brain employ some hugely efficient compression, trained on lots of real world data; and both these AI pictures and our dreams are sort of generated by "firing up" the compression machine from the other end with semi-random inputs.

So, yes, it's not wonder that they're alike in some ways, and disturbing, too.

Knowing this and that AI is so good at creating nightmarish pictures, would it be possible to attempt a reverse? Ask/learn AI to create the most frightening, most nightmarish rendering it can ever produce?

Is such picture even exist? That I would seen it once and wouldn't sleep for a week? Or upon seeing it, start crying without explanation?

Seeing something like this created by AI would be very impressive: "prepare to cry when you see this picture (guaranteed!)"

Have "fun" - http://nightmare.mit.edu/