Actually, you're close, Omnirizon: It would have been the tribe *itself* carving the huge phallus (aka cock-n-balls) on their *own* god-idol, *because* of it's deep divine meaning
The phallus remains not only a powerful symbol of physical potency, but one of the most powerful spiritual symbols, as well.
And, to add my 2c, it's my understanding that that particular chalk sculpture was likely carved by occupying Roman soldiers, to represent Herakleus/Hercules, who was commonly represented as shown.