Ages of art
history

I basically have respected a chronological order with this picture gallery, dividing all images into the following periods of art history:
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  Antiquity   until ~300 p.C.   Neoclassicism   1750 – 1830
  Middle Ages   100 – 1400   Romanticism   1800 – 1900
  Renaissance   1300 – 1550   Realism   1850 – 1890
  Mannerism   1520 – 1610   Impressionism   1860 – 1910
  Baroque   1580 – 1750   Symbolism   1870 – 1930
  Rococo   1720 – 1780   Modern Art   after 1900  

Copyright

All the images shown in this gallery are taken from the internet, mainly from Wikipedia.com, Web-Art.com.au and ocaiw.com.
According to those sources and the law within the USA, the European Union and Switzerland, all images are in the public domain and no longer protected by copyright.

More frontal
nudes

Images by artists who died less than 70 years ago cannot be shown here due to copyright restrictions. But you find many more examples of contemporary art images with frontal nudity on specialized web sites. I have listed some of them here with direct links for your convenience.