It's impossible to underestimate the contribution of Frazetta to fantasy art and to the commercial viability of fantasy as a genre in the first place, thanks to his covers for such authors as Edgar Rice Burroughs (Pellucidar and Barsoom) and Robert E. Howard (Conan) and others. Honestly, no matter how much the fantasy art field has blossomed in recent decades, and attracted some really skilled artists, nobody does it like Frazetta did.
Here's one of his book covers; I bought this book in high school in 1988 or 1989 for no other reason than because of the cover, of course. I went on to buy the rest of the series (still sitting in my unread queue at home, I might add... but I still pull them out and look at the covers from time to time.)
RIP, Mr. Frazetta. You'll be greatly missed.