Proclaim that mercy is the greatest attribute of God.
— Words attributed to Jesus by Saint Faustina in her diary.
The Divine Mercy devotion and it's venerated image refer to Saint Faustina's description in her diary of God's loving mercy towards all people of Earth, most especially the mercy towards sinners. The devotion has two major themes - The first is Christ's endless and boundless goodness and mercy and the second that receivers of this mercy should show mercy to everyone around them so that they may be a conduit of God's love towards the entire world.
The seven main portions of this devotion are:
The Divine Mercy image
The Commemoration of the Feast of the Divine Mercy Sunday
The Chaplet of the Divine Mercy
The Hour of Mercy (at 3AM or 3PM)
Spreading Mercy through words, deeds and prayers
The Spreading of Mercy to all of humanity in preparation for the return of Jesus Christ
In Saint Faustina's diary, she wrote that Jesus told her:
"I demand from you deeds of mercy, which are to arise out of love for Me."
These deeds of mercy can be accomplished in three ways - by deeds, by word and by prayer to one's neighbors. By utilizing all three, we are able to allow God's love and mercy to flow through our hearts towards everyone around us that is in need of God's love and mercy.
The image of Divine Mercy is venerated world wide. In her diary, Saint Faustina wrote of Jesus's words to her regarding this image:
“In the evening, when I was in my cell, I saw the Lord Jesus clothed in a white garment. One hand raised in the gesture of blessing, the other was touching the garment at the breast. From beneath the garment, slightly drawn aside at the breast, there were emanating two large rays, one red, the other pale. In silence I kept my gaze fixed on the Lord; my soul was struck with awe, but also with great joy. After a while, Jesus said to me, paint an image according to the pattern you see, with the signature: Jesus, I trust in You. I desire that this image be venerated, first in your chapel, and throughout the world. I promise that the soul that will venerate this image will not perish. ... I am offering people a vessel with which they are to keep coming for graces to the fountain of mercy. That vessel is this image. ... By means of this image I will grant many graces to souls" (Diary, 47-48, 327, and 742).
In the image, Jesus has one hand outstretched in blessing the viewer, with the other hand clutching his side that was wounded by the spear. Flowing out from this wound are two brightly colored rays - one blue and the other red, representing the blood and water that flowed out from Christ on the cross. Most depictions of this image also contain the words "Jesus I Trust in Thee". The more common painting was painted by Adolf Hyla. Adolf created the painting in Thanksgiving to God for having survived the horrors of the Second World War. The original was painted by Eugene Kazimierowski in Lithuania under the direction of Saint Faustina. She immediately asked Blessed Father Michal Sopocko to help find an artist for the painting and found Eugeniusz willing, if not fully understanding of the request. The original painting was completed in 1935.
While the original was beautiful, Saint Faustina began to cry upon seeing the finished work as she was sad that the depiction was not as beautiful as the vision she had received from Christ. In prayer, she wrote that Jesus came to her in her sadness and comforted her, saying:
"Not in the beauty of the colour, nor of the brush is the greatness of this image, but in My grace"
The Vatican banned devotion to the image in 1959. The Vatican questioned Saint Faustina's claims and began to grow uncomfortable with the image, as it's colors closely resembled the colors of the Polish flag. On April 15th, 1978, this ban was lifted by pressure from the future Pope, Saint John Paul II. Pope John Paul II as Pope would later institute the Feast of Divine Mercy Sunday in 2000, with the date set for the Sunday following Easter on the General Roman Calendar. Saint Faustina wrote in her diary that Jesus assured anyone who participates in the Mass, receives the Sacraments of Confession and the Eucharist on Divine Mercy Sunday of a full remission of their sins and punishments.
Saint Faustina's Diary is known as the Divine Mercy in my Soul and can be purchased in almost any language for further (and recommended) reading on both the Divine Mercy and her spiritual life. For US citizens, the book is available on amazon here.