ARTstor contains over 1.6 million high qualtiy digital reproductions of artwork from all major periods. Images can be downloaded and used in presentations for educational purposes.
In the past couple years, there has been evidence of a movement among some museums to digitize and make available high quality digital reproductions of artworks on the open web, mostly for academic purposes. Some of the works on these sites may be under copyright so while they can be viewed online, they may not all be available for reuse in other forms, such as presentations or research papers.
Works from the Met's collection. A great source for modern art, offering very high qualtiy digital reproductions, many of which can be viewed up close in fine detail.
The Google Art Project is a product of the Google Cultural Institute, referring to a partnership between Google and hundreds of museums, cultural institutions, and archives to digitize and host over 45,000 digital reproductions of the world's artworks.
In 2013, the Rijksmuseum in Amsterdam released 125,000 high-resolution digital images of works from its collection, encouraging others to download them and reuse them in creative ways. You can browse the collection by styles or search by artists' names to find modern artists of interest.
Open Content Program
The J. Paul Getty Museum and Getty Research Institute offer over 10,000 images to which Getty either owns the rights to distribute or are in the public domain. These images include paintings, drawings, manuscripts, sculptures, and lots more from the 16th-18th century. It also includes 19th century architectural drawings of cultural landmarks.
While not an ideal source for the modern art period, it is still a great source for art history students.