In Roman Catholic teaching, "original sin" is the sin of Adam which humans inherit at conception but which is washed away in Baptism. "The Immaculate Conception" is the doctrine that Mary was preserved from original sin at the moment of conception, receiving in advance the grace of Baptism that her son would earn for all mankind. Writers since the 4th century have referred to Mary as "immaculate" but the doctrine was officially promulgated only in 1854 (Catholic Encyclopedia, s.v. "Immaculate Conception").
In the 16th century Molanus (393-94) commended the use of references to the Song of Solomon in images relating to Mary's conception. He mentions the sun, moon, stars, and the words of Canticle 4:7, Tota pulchra est amica mea et macula non est in te ("All beautiful is my friend and there is no stain in thee"). In reference to this verse some Immaculate Conception images are entitled Maria Tota Pulchra. The second one on the left is an example.
In the 17th century a specific iconography for the Immaculate Conception developed from what was taken to be a reference to Mary in Revelation 12:1 – "And a great sign appeared in heaven: a woman clothed with the sun, and the moon under her feet, and on her head a crown of twelve stars." As in the picture at left, the images show Mary "clothed" with a sunburst, with twelve stars about her head and the moon beneath her feet.
Sometimes there will be a dragon beneath Mary's feet, in reference to the red dragon of Revelation 12:4 that "stood before the woman who was ready to be delivered, that when she should be delivered, he might devour her son." In the second picture on the left, the dragon is being cast down by the archangel Michael, which is what happens in Revelation 12:7-9.
Angels are usually in attendance. The moon may be a sphere, as in the first picture at left, but it is more commonly a crescent, as in the third. The fourth has Mary standing on clouds, but this is less common.