Optical Illusions.
And not the ones about water falling/going up or the ones where you stare at a few simple geometries and then close your eyes to notice an after-image burned in your retina.
I mean the ones where a face is a actually a clever assembly of unrelated imagery.
Check the Wikipedia article about death and you will find a painting of two beautiful girls which also happens to be a gigant smirking skull staring at you, that ************.
Or even better (worst), look for Hans Bellmer's art on the web. The mannequin monsters in Silent Hill 2 were directly inspired by his photographed sculptures. His drawings are way creepier.
I once stared at a painting of a woman who just happened to be a milion images sewn together, and it overwhelmed me to a point that all sights and sounds around me vanished for a few seconds.
Funny, because when I was a kid they posed nothing more than a curiosity to my eyes.
Don't even get me started on that Magritte "portrait" of the blond woman...