Columbia's Avery Art & Architecture Library has over 625,000 volumes of art and art-related books. None of Avery's books are allowed to be checked out, which means they'll generally be easily available to browse. Search for Avery's books in CLIO, or browse their stacks.
CLIO (Columbia Libraries Information Online) is the online catalog of the Columbia University Libraries, including Barnard Library, but excluding Teachers College and the Law Library.
Digital museum collections are a great place to find high-quality reproductions of works of art, as well as all the information you need to properly caption your images. Below you will find a selection of museums (and a couple of libraries too), which have strong digital collections online.
Have an image and can't tell where it's from? Try using a reverse image search to find out what it is!