One of my favorite sites to visit for rare images is an auction site called mutualart.com. I first learned about the site while I worked at Sotheby's and I've been following it ever since.
The reason it's a great site is because you can find images on here that you'll never see anywhere else. The images also include the dimensions and medium of nearly every work. There's a feature that allows your to receive auction updates for your favorite artists. The updates include events, articles and research pertaining to your selected artists.
It's free to sign up and there's some kind of upgraded section that allows access to further auction records. For my purposes, the basic account is great. There's only a few downsides to the site: some of the images have terrible quality and the site contains a lot of clearly fake, imitation, works.
For art history nerds, like me, it's also great to do research on the academic process. For instance, I just spotted this Bouguereau study that I recognized as being a study for another unfinished painting. I discussed the unfinished painting in this post.
From a technical point of view, finding studies like this helps tremendously; you can learn so much by studying them. It's hard to come by images that display the process and that's why I love this site.