Da Vinci's: http://www.museoscienza.org/english/leonardo/
http://www.ibiblio.org/wm/paint/auth/vinci/
http://legacy.mos.org/leonardo/
http://www.gutenberg.org/ebooks/5000

Renaissance Art at Khan Academy:
http://smarthistory.khanacademy.org

David:
http://smarthistory.khanacademy.org/Michelangelo-David.html
http://www.pbs.org/wgbh/cultureshock/flashpoints/visualarts/david_a.html

La Primavera:
http://www.uffizi.org/artworks/la-primavera-allegory-of-spring-by-sandro-botticelli/

The Last Supper:
http://smarthistory.khanacademy.org/leonardo-last-supper.html
http://www.italianrenaissance.org/

The Mona Lisa:
http://smarthistory.khanacademy.org/leonardo-mona-lisa.html

School of Athens:
http://smarthistory.khanacademy.org/school-of-athens.html

Creation of Adam:
http://www.learner.org/courses/globalart/work/78/index.html
http://www.italianrenaissance.org/?s=creation+of+adam&x=0&y=0

Madonna of Chancellor Rolin:
http://www.louvre.fr/en/oeuvre-notices/virgin-chancellor-rolin

Marriage of the Virgin:
http://smarthistory.khanacademy.org/raphael-marriage-of-the-virgin.html

The Chess Game:
http://www.wga.hu/frames-e.html?/html/a/anguisso/sofonisb/chess.html