The Art Institute of Chicago has recently enhanced the online availability of its digital archives, making much more of its collection available for online viewing. More than 44,000 images of its collection are now available online with unrestricted access. The images are all downloadable under a Creative Commons license. Image viewing has also been enhanced, allowing the user to see the image in much greater detail, even down to the brushstroke level.
Explore the collection here. You can browse through varies genres or use the search function to look for more specific pieces.
To find out even more about the collection of the Art Institute check out these books from the MVCC library.