'the strongest' refers to on the one hand the lion in
the picture. Cupid is clearly on top of the creature,
holding it with reins. The normally ferocious lion has
a sullen, submissive look on it's face. On the other
hand, 'strongest' refers to the most stubborn of human
beings, of whom is implied does not sumbit to anyone or
anything. Even he will be tamed by the power of love.