Art Magick-This is many AW people's favorite art hangout, and is one of mine as well. Some of the great things they offer:
Painting of the Day ; some of my favorite artists I discovered from these
Art Galleries ; If you are looking for say, all pre-raphealites, fairies etc, they have inclusive galleries of these artists.
Art Albums ; This is a feature only if you register, but registration is free, and this is an awesome feature! I have 4 already, two for artists of the avatars, one egyptian art, and one Knights and Ladies of Legend.
Thumbnail gallery veiw ; lets you see all their works in mini form so you can see right away what you'd like to view larger.
Also contains large fairly good resolution images if you click on regular sized ones.

ARC -This gallery is absolutely huge, and contains much more in depth amount of each artists works of art. For instance, the artist currently highlighted in my Art Portal (in domus) is on Jean Leon Gerome, who's gallery in Artmagick.com only contains maybe 1/5th the amount of art that this site contains. Also has Modern Era Artists and high resolution downloads (and these are HUGE). Unfortuantely, it also takes absurdly long to load each page, and I use ADSL line so it's got to be unbearable at dialup speeds. Also, dont even bother looking at their registration/membership pages, they have one on offer for 10,000 US dollars. What the!

Alternate Realities in Art and Thought-Excellent classical and modern artists gallery, they have some fantastic images from the Book of Kells and others. Their modern artists galleries I would inquire about permission though before doing any serious altering to the images, like one would do for avatars etc. In fact, some of the galleries ask that you do not copy the images at all without artist permission. Though that is fairly easy to achieve from what I understand.

Fantasy Art by Selina Fenech-excellent modern artist, though obviously only good for fantasy art (fairies, goddeses etc) She has a very fair terms of usage located in a link from that page above, and allows her art (except the 'exclusive pieces') to be used on the net as long as it's commercial free and not altered.

Olga's Gallery-A long established and very good online gallery of classic artists. Updated very often (still) and offers indepth topics in art lists, such as Greek Mythology. (no Bellerophone though?? Or Chimera,hmm)