Some suggested pictures:
Botticelli: The Adoration of the Magi. National Gallery of Art
Buoninsegna, Duccio di: The Temptation of Christ on the Mountain. The Frick Collection (New York)
Campin, Robert. The Annunciation Triptych. Metropolitan Museum of Art: The Cloisters
Eyck, Jan Van: The Annunciation. NGA
Filippo Lippi: The Annunciation. NGA
Fra Angelico and Filippo Lippi: The Adoration of the Magi. NGA
The Unicorn in Captivity. MMA:C
Veronese, Paolo. Allegory of Virtue and Vice (The Choice of Hercules). TFC

For a ready supply of slides available in sets or as singles: www.davisartslides.com

For access to museum shops, prints, and posters:

Art Institute of Chicago www.artic.edu

Metropolitan Museum of Art www.metmuseum.org

Museum of Fine Art, Boston www.mfa.org/shop

National Gallery of Art - This is one is the best, including narratives of paintings that you can view on screen, some in detail. www.nga.gov

Other resources:
www.groveart.com