Why do pictures of Cupid also appear on valentines?

Cupid, also known by his Latin name Amor (you may recognize this as the stem of the Word "love" in Romance languages), is the Roman god of erotic love and beauty. He is the son of Venus, the goddess of love, and Mercury, the messenger god. While this line of logic isn't documented, one can assume that a mixture of his parents roles, love and messenger, makes for a convenient appearance on Valentines Day cards.