The portrait was uncovered on the wall of a chapel in Domremy, in eastern France, Joan's birthplace.
Experts engaged by the French culture ministry are trying to establish whether the picture is a portrait of the young woman painted while she was still alive.
It was in Domremy that Joan - properly speaking Jeanne d'Arc - claimed to have heard the voice of God urging her to rally her countrymen against the English invader.
According to local tradition, she often prayed in the chapel where the picture was found. It shows a young woman in characteristic peasant headware, kneeling besides a saint, Thiebaut de Provins.
The local bishop asserted last week that the picture was definitely Jeanne.
It was discovered when workmen restoring the church removed a layer of lime- wash spread on the walls during an outbreak of the bubonic plague in the 16th century.
The painting has been definitely dated to the early 15th century, when Jeanne was alive. After inspiring a successful campaign against the English, she was betrayed by French enemies to the invaders, who burned her at the stake as a witch in 1431, at the age of 19.
Jeanne remains one of the great heroines of French history; her reputation has even survived energetic efforts by the far-right National Front to turn her into a symbol of ultra- nationalist resistance against foreigners.