I came to D&D late in my gaming career, only acquiring a copy of the AD&D Players Handbook last year, at the ripe age of 27. Thus this image, while dazzling to my adult self, didn't have near the impact that it would have had on my 12-year-old self had I seen it back then.
Anyway, this is the image that says "D&D" to me. Part of me wants to yell "Don't look at that freakin' mouth, there's some glowing eyes at the end of the hallway! Get your shields up!"
Anytime I want to get juiced up for fantasy gaming, I take a look at this image. And whenever I need to get in the right mindset for D&D in particular (what with its adventuring parties, hirelings, 10-foot poles, secret doors and the like) I take a look at the following image.