Elfwood () is a collection of fantasy art (and some science-fiction). The art is copyright to the creators, but you can always get the email address of the artist and contact them asking for permission. My experience is that you get permission about 50% of the time. The quality ranges from excellent to woeful - one thing that helps is putting 'modpick' in your searches, this means that all your results will be Moderator's Picks ie hopefully of a higher-than-average quality.
DeviantArt () tends to have higher-quality artists (it seems that a lot more professional artists are on there) but I've gotten a lower response rate.
Also, a lot of art in the public domain can be easily used for fantasy: particularly Surrealist, Pre-Raphaelite and Orientalist art.
I think the art is the best thing about my site actually.