The Sacred Heart is the image of a stylized heart, surrounded by a ring of thorns, and surmounted by flames. Often, the Sacred Heart is also wounded and bleeding, and surrounded with light. The image represents the power of Jesus' divine love: the heart itself the ultimate symbol of love, surrounded by the crown of thorns with which He was made to suffer, inflicted with the wounds He received while on the cross for love of his people, burning and radiant with love.
It is believed by some that Jesus appeared to Saint Margaret Mary Alacoque in 1672. The story states that Jesus explained to her his desire to see people practice devotion to His Sacred Heart, and that he wished to be represented (honored) through a symbol of His literal heart.
Its appearance as a tattoo icon appears to have started with Cholo/Barrio fineline tattooing and was prevalent in old school tattoos. Recently has been appropriated by the "new skool". In addition, people have been superimposed onto Sacred Heart style portraits of Jesus on at least one occasion. Some believe this to be sacrilegious while others find it a to be a tribute or a declaration of faith.
A similar image is that of the Immaculate Heart (or Sorrowful Heart, or both) of Mary, Jesus' mother. The Immaculate Heart, representing both the physical and spiritual the purity of the Virgin Mary, is surrounded by flowers and is also burning with Her love for Jesus and His people. The Immaculate Heart was used extensively by Baz Luhrman, along with much other religious imagery, for the film Romeo + Juliet.