Many museums are making their collections freely available to the public via the museum websites. Many of these are also open access so without copyright and usable for any purpose. It is advisable to cite the source of any image you insert into your paper or publication, including Artist, name of work, museum and website where it was obtained.
If the image is designated as copyrighted, you must ask permission before publishing it. Museums and other institutions will often allow permission and provide hi res files for no or low cost, as opposed to stock image companies such as Art Resource or Getty Images/Corbis, who often charge much larger fees, but offer a quick turn around and image researching services.
The College Art Association has a webpage on image use. They also have a Code of Best Practices which provides visual-arts professionals with a set of principles addressing best practices in the fair use of copyrighted materials. It describes how fair use can be invoked and implemented when using copyrighted materials in scholarship, teaching, museums, archives, and in the creation of art.
To quickly find an image, try using Google Arts and Culture
Or try looking through the blog Free Academic Images
Or try looking through the many open source Art History Projects listed on The Digital Art History Directory
The Rijks Museum in Amsterdam has made most of their collections available freely online. Users can explore the entire collection, which is handily sorted by artist, subject, style and even by events in Dutch history
Images d'Art (Art Images) gives you access to a rich database of 30,000 artists and hundreds of thousands of their works that you can discover, share and collect.This image database contains works from French museums that have been digitized and documented by the RMN-GP’s photo agency.
From the website: Explore thousands of artworks in the museum’s wide-ranging collection—from our world-renowned icons to lesser-known gems from every corner of the globe—as well as our books, writings, reference materials, and other resources.
The Metropolitan Museum of Art Collections Includes spotlights on key works and collections, open access art images and exhibit information
Art Resources from the Mid-20th Century from the website: This collaborative project between the Solomon R. Guggenheim Museum and the Whitney Museum of American Art, presents digitized highlights from the personal libraries of Hilla Rebay and Juliana Force, the museums respective inaugural directors. Digitized publications from both libraries can be found on the Art Resources Project Page at the Internet Archive with highlights from Hilla Rebay’s collection shown below.