The highlight of the chapel is the painting "Our Lady of St Michael", which is more usually called "The Madonna of the Sapiehas". This image of the Blessed Virgin Mary with the Franciscan saints Francis and Bernardine has been famous for its miracles since the 16th century. Formerly kept in the Bernardine Church, the painting later belonged to the Bernardine convent’s Church of St Michael the Archangel. The name "The Madonna of the Sapiehas" refers to the name of the family who endowed the convent. Miracles worked in the presence of the painting were recorded, and on 8 September 1750 the image was decorated with crowns sent by Pope Benedict XIV (the frame and the crowns have not survived). It was the fourth holy image in the Grand Duchy of Lithuania to be crowned (and the second oldest in Lithuania today).

In 2020, through the efforts of the Cathedral community, the crowns of Our Lady and Infant Jesus were restored and adorn the painting again.