Arnie Fenner, co-founder and editor of the Spectrum collections of fantastic art, has written an article for Tor.com titled Lovecraft, Asimov, GRRM, Heinlein & More: Painting SFF Writers in which he collects some portraits of fantasy and science fiction authors by a number of artists.
Covering authors in the genre as far as Mark Twain (think Connecticut Yankee in King Author’s Court) the portraits are in a variety of media and styles, but all by outstanding artists.
Incidentally, the Tor.com site has a terrific gallery of fantasy and science fiction illustrators (major time sink warning).
(Images above: H.P. Lovecraft by Virgil Finlay, Robert A. Heinlein by Donato Giancola, Edgar Allen Poe by Michael J. Deas, Harlan Ellison by Iain McCaig, Jules Verne by Mark Summers, Mark Twain by Gregory Manchess.)