Here are some of the most iconic paintings in art history from the Renaissance up to the year 1900:
"Venus of Urbino" by Titian:
http://medializzy.files.wordpress.com/2009/05/titian_venus_urbino.jpg
"Girl with a Pearl Earring" by Jan Vermeer:
http://www.expasy.ch/spotlight/images/sptlt112.jpg
"Las Meninas" by Velasquez:
http://england.tasis.com/uploaded/News_Stories/Spanish_Art/velasquez-1656-las-meninas.jpg
"The Swing" by Jean-Honore Fragonard:
http://amysbabies.files.wordpress.com/2008/03/frag_swing.jpg
"The Raft of the Medusa" by Theodore Gericault:
http://www.artandperception.com/v01/wp-content/uploads/2007/10/gericault-raft_of_the_medusa.jpg
"La Grande Odalisque" by Ingres:
http://art-quarter.com/beck/joe/aj/1/3/ingres-odalisque98.jpg
"Olympia" by Manet:
http://www.griseldaonline.it/foto/checcoli/1D%20-Edouard-Manet-Olympia-1863-parigi-museo-d%27orsay.jpg
"Starry Night" by Vincent van Gogh:
http://www.wagner.edu/departments/art/sites/wagner.edu.departments.art/files/images/Van_Gogh_Starry_Night.jpg
"The Lady of Shalott" by John William Waterhouse:
http://users.skynet.be/michaelssite/Pictures/waterhouse.jpg
"Sunday Afternoon on the Island of La Grande Jatte" by Georges Seurat:
http://en.wikipedia.org/wiki/File:Georges_Seurat_-_Un_dimanche_apr%C3%A8s-midi_%C3%A0_l%27%C3%8Ele_de_la_Grande_Jatte.jpg
...and many, many more.