Many of the search engines have gathered together what they term "galleries" or "directories." This is a nod to the success of Yahoo which categorizes links into like-minded groups. Here are some that bring together galleries of SF and fantasy art including collections related to particular artists.
DMOZ Open Directory
Google Web Directory

Halls of Tolkien

With the release of Tolkien's The Hobbit, his publisher has included art done by the author either as inner pieces or on the dust jacket. It has proved a popular addition to many of Tolkien's books. Indeed, there is a small industry devoted to producing art for the annual Tolkien Calendar. Here, you can find a huge Tolkien-based art gallery.

Hieronymus Bosch

Perhaps there has been no greater influence on the development of fantastic art than Hieronymus Bosch. Clues and hints of his work have appeared in the work of most of the artists working today in fantasy and science fiction. Sceptical? Grab a book of Bosch's work and spend some time comparing it to some of your favourite contemporary SF and fantasy artists.

Dreams Studio

All the graphics you can find there are free. You can use them in your programs, Web site or any software you like as long as you respect the restrictions which apply in the copyright agreement. It is meant to be on-line source of illustrations.