Search the collections of the National Gallery of Art: https://www.nga.gov/collection.html
Search the collection of the Museum of Modern Art: https://www.moma.org/collection/
Metropolitan Museum of Art (click on the magnifying glass to search the collection): https://www.metmuseum.org/
Search the collection of the Louvre Museum in Paris: https://www.louvre.fr/en/recherche-globale
Search by artist name or art movement along the left-hand side of the page:
http://www.artchive.com/ftp_site.htm
Artchive: http://www.artchive.com/