Lilies are frequently found in paintings of Our Lady, because the lily is one of the flowers that has traditionally represented her, and her attributes, as explained in the following:
The lily is a symbol of purity, and has become the flower of the Virgin. Originally, in Christian symbolism, the lily was used as the attribute of the Virgin Saints. The lily among thorns has become a symbol of the Immaculate Conception of the Virgin because of the purity she preserved amid the sins of the world. The Annunciation, is very much associated with lilies. In many of the scenes of the Annunciation executed during the Renaissance, the Archangel Gabriel holds a lily, or a lily is in a vase between the Virgin and him. Thus, the lily is also an attribute of the Saint Gabriel. source
The great St. Gertrude (b. 1256- d. 1302), who received signal favors from God, was blessed to receive an apparition of Our Lady wherein Our Lady expressed her wish to be honored in reference to the lily. The following is an excerpt from The Revelations of St. Gertrude, Book 3, Chapter 18:
The following day, as Gertrude prayed, the Mother of God appeared to her, in the presence of the ever – adorable Trinity, under the form of a white lily, with three leaves, one standing erect, and the other two bent down. By this she understood that it was not without reason that the Blessed Mother of God was called white lily of the Trinity, since She contained in herself, with more plenitude and perfection than any other creature, the virtues of the Most Holy Trinity, which she had never sullied by the slightest stain of sin.
The upright leaf of the lily represented the omnipotence of God the Father, and the two leaves which bent down, the wisdom and love of the Son and the Holy Spirit, to which the Virgin approaches so nearly.
Then the Blessed Virgin made known to her that if anyone salutes Her devoutly as the white lily of the Trinity and vermilion rose of Heaven, She will show how She prevails by the omnipotence of the Father, how skillful She is in procuring the salvation of men by the wisdom of the Son, and with what exceeding love her heart is filled by the charity of the Holy Ghost. The Blessed Virgin added these words: “I will appear at the hour of death to those who salute me thus in such glory, that they will anticipate the very joys of Heaven.” From this time the Saint frequently saluted the Holy Virgin or her images with the below words:
“Hail white lily of the ever peaceful and glorious Trinity!
Hail, effulgent rose, the delight of Heaven,
Of whom the King of Heaven was born and by whose milk He was nourished! Nourish our souls by the effusions of thy Divine influences.”
–(Known as the ‘Hail Mary of Gold’ prayer) source
Hail Mary, hail white lily!