I think this depends on what type of images fall under dreaming and nightmare. From my understanding of the dreaming tag, the dream has to be depicted. If that is true, then it depends on what can fall under the nightmare tag. Does the nightmare tag cover just images in which you see the character is dreaming and having bad dreams, or would a depiction of a character sleeping but with a pale face, tossing & turning, and murmuring things like "noooo...." also qualify for the nightmare tag?
It should also be noted somewhere the ambiguity of the nightmare tag given how there are a few characters/creatures called "nightmare."