At their evening auctions of Impressionist and modern art in May , both companies are selling historically important portraits : at Christie 's , an 1890 van Gogh image of Madame Ginoux , the proprietress of the Cafe de la Gare in Arles , France , which the artist frequented , and at Sotheby 's , a seminal 1941 Picasso portrait of his mistress Dora Maar that has not been seen in public for more than 40 years .