So, so many choices. I felt it was a little inappropriate to pick something because was particularly titillating or something that was really reflecting a licensed product (so no Star Wars illustration, for example). There are a lot of classics out there, but the one I first thought of was this one.
It's from the 1st Ed. DMG. I love the photo because it seems to encapsulate what I feel like fantasy RPG's should be about, namely average people having to overcome really freaky stuff. I'm someone who at heart likes "fantasy horror" over "murderhoboing" or whatever fantasy roleplaying tends to be these days. Also check out the glowing eyes at the bottom of the step. I love a picture that tells a story and isn't just some awesome looking hero with a heroine slumped in his arms as he rides a dragon or some such thing.
It also, ironically, shows what it sometimes wrong with fantasy RPG's, in that many people will look at the photo and not wonder, with real wonder, what it is but instead say "it's three dwarfs, a halfling, and a magic mouth spell." I think that one of the real pitfalls for fantasy RPG's, and any speculative fiction-based RPG's, including horror or science-fiction, is that when you have the rulebook accessible to all the players, the wonder and awe vanishes. It's the difference between telling your players that they are being confronted by a vaguely human-looking creature with green skin covered with pustules, lank greasy hair, yellowed teeth, and chipped claws versus telling them that they are facing a troll. Oh, a troll? Well fire will take care of that guy. *heaving sigh*