We are, of course, the sum of our experiences. When it comes to how we view fantasy, I think we are each a crucible in which our influences are made molten and then shaped into something old-yet-new again. The early influences are at the core of our imaginative alloy.
I've been tracing the early fantasy visuals I was exposed to and attempting to unravel where each fits into how I imagine. How do you judge what was an important influence? This is my (undoubtedly shoddy) rubric: if you look at it now, you still feel a visceral reaction to the possibilities it hints at. These are the early influences I still find wonderful, and what I think they taught me.
As near as I can tell, the run of the "I...Vampire!" story in The House of Mystery was my first exposure to a grotty, lurid Gothic aesthetic. Get obsessed, and stay obsessed.
My first D&D book wasn't a game book at all. Instead, it was The Forest of Enchantment AD&D storybook I bought from the school book sale in elementary school. This scene of ren faire bards & druids vs. sword & sorcery warriors and wizards set an important tone: in fantasy, anything can be mixed. Do not bat an eye; do not cry "milieu!"
We didn't get the paper at my house when I was a kid, but when I was at my grandparents' house I would try to piece together the narrative of Prince Valiant comics from whatever Sunday papers they had forgotten to throw out. There would be gaps in the story, of course, but that didn't make my interest wane at all. I still believe that it's okay to have "gaps" in your game's story.
The mini-story books that came with the first bunch of He-Man toys were also terribly captivating. The cartoon was a massive sanitized disappointment after the weird sword & sorcery aesthetic these comics deployed. It seemed like the creative team didn't feel the need to check their weirder impulses: a skull-faced would-be conqueror? A barbarian on a giant green tiger? A space cop entering the fray? Let weirdness be your permission slip.
Speaking of He-Man, I could spend all day looking at this decal for the dungeon of Castle Greyskull and wondering what each of those beasts entailed. The monsters you see are only half the story; there are also the monsters you never see fully--those are the ones that stay with you.
Before I read Tolkien or C.S. Lewis, my mother's friend gave me a set of Lloyd Alexander's Prydain books. This was my first real introduction to Celtic myth; it's brilliant to steal from and re-shape myth.
That same friend also gave me a tarot deck illustrated by Palladini. I still have it. Incorporating resonant symbols is a useful shorthand.
The Dragon's Lair game was always broken and unplayable at Chuck E. Cheese, but that didn't stop me from watching the demo loop over and over again. It was all action scenes, really; remember action and keep things moving.
The Sorcery! books were the intermediate step between Choose Your Own Adventure and D&D for me. They were chock full of grotty, weird John Blanche art. These were definitely the gateway drug that led me to play Warhammer in high school. Sometimes it's okay for your character to die horribly--as long as that is entertaining, you'll be moved to start again from the beginning. (It's also okay to fudge things to get to the end!)
My aunt gave me the Scary Stories to Tell in the Dark books. The art by Stephen Gammell is utterly amazing. I'll never get over this stuff. The pictures are all from well-worn campfire tales--but even though the shape of those stories aren't surprising, the art does surprising things with them. You can do new things with old ideas.
The art from my first Lovecraft books. I love the limited palette. It's liberating to do a lot with a limited pool of color.
The summer I started playing D&D was also the summer I was reading Moorcock's Elric books. In my mind, D&D was mystical and hazy and effete and decadent like the covers of these novels. Escapism is one of the finest things.