I also like www.allposters.com and www.art.com - many times I will have my students start with these two sites, and then move to artcyclopedia for more detail on their chosen artist.
I like this site too. My students enjoyed searching the internet for images...rather than using their books...and of course, the images on the internet look much better and more colorful than the ones in our books.
My favorite source of digital images is www.artcyclopedia.com. It's where I start for all of my fine arts needs. I think it's better than all of the other sites combined.
I can't be the only one on this exchange who's used this site. Yet, I haven't seen any mention of it from members. You guys know about this, right? If you have found any problems with Artcyclopedia, please let me know. I've been using it for about two or three (?) years and I've never had a problem. But, it can't hurt to ask.