This post over at Grognardia seems to have spawned a meme, as a number of gaming bloggers have posted their responses. Here's mine.
This is the cover image that, more than any other, makes me think of Dungeons & Dragons:
This image was plastered over the comics of my youth, so before I even had the slightest idea of what a roleplaying game was, I was aware of D&D. As such, this image has a lot of nostalgic pull for me, although not enough to make me pick up the the new edition of D&D4 masquerading under this image.
Still, even this isn't the image that defines D&D for me. That image isn't a cover at all, and it's arguable if it's even an image as such:
To this day, I have a preference for landscape character sheets in D&D, purely because of this one document.