I don't make comics, so there's not much illustrated ones (I believe making readers imagine it themselves will make it scarier)
While not nightmare fuels per se, I kinda have some unsavoury looking creachers (and humans) drawn for my own reference.
spoiler because long

Cervitaur avatar

The human behind it


Harpy avatar

Hare beast
My series itself has people mistaken its direction to be supernatural horror, but it's ultimately not (and IMHO not scary at all). I also have toned it down compared to the draft versions.
I really like using nightmare or hallucination scenes, it can be an easy way to portray the characters' state of mind and foreshadowing.
Straight into the second chapter, the protagonist saw a fetus floating in his soup. I add recurrent imagery about things related to gestation and birth (placenta, deformed fetus), because it has significance in the future and I just find fetuses uncanny.
If I was to be honest, I find the power system and societal aspect of the story are more disturbing. I haven't gone too far in the story to make it apparent (and probably readers miss it anyway, because I don't make it a focal point), but so far we have curses, oppressive governments with their "moral correctness" plus brainwashing programs (and they can insta-kill you in some instances!), cults, shitty "afterlife", human trafficking, people being turned into objects, and cannibals.