Man, oh man. This is a tough one. I’ve seen hundreds of pieces of fantastic and evocative art from Larry Elmore, Keith Parkinson, Erol Otus, and others that formed the basis of what I think of when I think of fantasy. Later came the art for sci-fi RPGs and cyberpunk, post-apocalyptic, and other genres. I can’t really pick just one as my favorite illustration (what I like in art is greatly affected by my current mood, as well).
I like art that puts so much detail in, you can always find something you didn’t notice the first dozen times you look at a picture. Like this red dragon by the late, great Keith Parkinson (whom I was privileged to meet at the last Gen Con he attended). I had this as a poster in my room as a teen and didn’t notice the ship in the background for months.