The Library of Congress has loads of amazing images online, and is adding more regularly (see Exhibitions page). Here are just a few of the many to consider when putting together courses.
The Empire That Was Russia: The Prokudin-Gorskii Photographic Record Recreated is an amazing collection of early twentieth-century color photographs from Czarist Russia. Prokudin-Gorskii traveled all over the empire photographing its inhabitants using a system of three cameras with separate filters. This would be great for use in history of photography courses and for Russian art.
Online exhibition on Leonardo's Study for Adoration of the Magi.
Arthur Szyk: Artist for Freedom. Work by a Jewish antifascist cartoonist and miniaturist, mostly around the time of World War II.
The Floating World of Ukiyo-E: Shadows, Dreams, and Substance. "This exhibition showcases the Library's spectacular holdings of Japanese prints, books, and drawings from the 17th to the 19th centuries. These works are complemented by related works from the Library's collections created by Japanese and Westerns artists into the 20th century."
Frank Lloyd Wright: Designs for an American Landscape, 1922-1932.
A Heavenly Craft: The Woodcut in Early Printed Books.
The Work of Charles and Ray Eames: A Legacy of Invention. Showcases the designs of this legendary team. Lots of images.
Life of the People: Realist Prints and Drawings from the Ben and Beatrice Goldstein Collection, 1912-1948. Includes works by Sloan, Benton, and other American artists of the first half of the twentieth century. This would make a good supplement to Frances Pohl's Framing America text.
The Library of Congress has many more exhibitions, plus images such as early photographs and daguerreotypes, early sheet music covers, and so forth.