When I was standing in front of this enormous painting, I was confused. On the one hand I recognized art nouveau, but the colors used did not match with that period. The colors and figures were so typically Greek to me, that I thought it was a weird mixture of styles.
Konstantinos Parthenis, the creator of this painting, is one of the first Greek painters who brook with the Greek academic tradition of the 19th century. He was the first to use modern elements in his paintings. His father was Greek, but he was born in Alexandria and lived and studied in European cities like Paris and Vienna. He came to Greece for the first time in 1903 and lived there for 5 years. After a stay in Paris for another 5 years, he returned to Greece permanently where he lived until his death in 1967.
On the website of the National Gallery, where you can find this painting, is said that: Parthenis is one of the most important figures in modern Greek art, both for his work and for his contribution through teaching. In his painting, which includes religious subjects, landscapes, mythological and allegorical motifs, portraits, and still lifes, drawing his inspiration from ancient and Byzantine art but also from modern currents – impressionism and post-impressionism, symbolism and Art Nouveau – he fashioned a completely unique and personal style, with which he transformed his ideas and visions into images, opening the road for the revitalization of Greek art.
This painting, the apotheosis of Athanasios Diakos, was painted in 1933 and is now hanging in the National Art Gallery in Athens, Greece. On their website you can also find more examples of his work and a short description of his life.
Athens has many more modern art museums besides the National Gallery. Read my article about modern art in Athens published at Kapelkatravel.com (in Dutch, but you can google-translate on the website for a translation into your own language).