You would think that there wouldn't be enough bandwidth on the Internet to sufficiently explain that image in a way that lets it make sense, but we'll try. Keep in mind, religious artwork doesn't necessarily have to be influenced directly by the Bible. Sometimes it can be based on legends and folklore while still remaining true to the fundamentals of church art, like being as uncomfortably wacky as possible. Case in point: the legend of St. Bernard of Clairvaux.
The patron saint of Rule 34.
There are two versions of the St. Bernard legend, both of which concern Bernard receiving a vision where he gets to sample breast milk from the Virgin Mary, whose impressive aim has been seriously downplayed in the Scripture. One version of the legend claims that the Virgin Mary let Bernard sample her milk to show him that she's humanity's mother and can totally put in a good word with Jesus, so we should probably pray to her, too. In the other version, the milk was meant to impart the heavenly wisdom of God, but this version also plays heavily on the motif of motherhood.
The original MILF (mother imparting liquid faith).