The Telegraph explains how changing mores and tastes resulted in changes in the pictures at the National Gallery:
When the gallery acquired the Renaissance painting Woman at a Window in the 19th century, the painting depicted a modest young brunette woman looking out from behind a curtain. A black and white photograph records the picture in this condition. But following an X-ray by the gallery’s scientific department which showed that the painting had been modified […] The picture, which is by an unknown artist and dates from 1510-1530, had been drastically altered in the 19th century to satisfy more decorous Victorian tastes.
Betsy Wieseman, the curator of Dutch paintings at the National Gallery and co-curator of the exhibition, said of the p[ainting: “It’s a remarkable document of changing artistic tastes and appreciation over the centuries. The modifications, which include airbrushing out her nipples, were probably made in the 19th century to suit more chaste and decorous Victorian tastes. The original woman looking out of the window is a sultry femme fatale, which would not have been deemed an appropriate image to hang on Victorian walls.”
What Lies Beneath (Telegraph)