As he was a Giant man, he helped people cross the river by carrying them on his shoulders. Anyways, as you might have guessed, carrying this kid made him so physically exhausting and the river turned unusually violent lending the child to reveal himself as the infant Jesus.

Hence he got the St. Christ(opher- meaning carrier).
Anyway apart from the mythology apart, this painting is done in the fresco technique by the famous fresco master Domenico Ghirlandaio. In this technique, pigments are mixed with water and applied to the fresh plaster made on the wall. As the plaster sets, the chemical reaction makes the plaster become part of the wall. Also, since the painting can only be done on the fresh plaster, the artist has to divide his painting into sections and plan to finish each section in one sitting. This division can usually be made out on a close up view. Fresco paintings have to be transported along with the wall that it was painted on.
Did you notice a flaw in the painting?
Look at his legs above and below water. May be in the mythological books, light does not refract on the surface of water ;)