That picture from the Spelljammer boxed set of the "pirate" guy standing over a defeated (but not dead) illithid, with a ship in the background. Really captured the feeling of Spelljammer, for me.
The cover of the Council of Wyrms hardcover... really evocative of the book as a whole (I must be one of the few people who LIKED that book...)
Just about any of the full colour art for Dragonlance... That has got to be the line blessed with the best artwork in DnD history. IMO, of course, as I'm not a huge fan of the planescape art. A few specific ones, though, would be the "Death of Sturm" one (That picture still can bring a tear to my eye), the "Lord Soth's Ride" one, and the cover of the novel "Soulforge", showing a young raistlin, still with dreams of power and respect, but not yet tainted by evil.
Not exactly DnD, but it's a WotC fantasy RPG... the picture of a woodsman striding forth boldly from a misty forrest in the WoT hardcover, with mountains just peaking the trees in the distance, intrepid look on his face... It's used as a chapter illustration, I wanna say chapter 6, but I'm not sure off hand. About midway through the book, I think. That picture so captures, to me, what it should mean to be a ranger-type character.