This painting can mean many things.
Conquering your fears, obstacles, challenges, and or hardships in your life.
David was a boy facing an Unstoppable Warrior and many Doubted him. Painting David as a young boy would only mean he only had his faith and trust in God to conquer his fear to defeat this opponent. He wasn’t strong, fully developed as most men. He was only a boy.
The Tears of Triumph ran down his face as He realized he Defeated Goliath and proceeded to show Everyone including his enemies what he had conquered.
48. So it was, when the Philistine arose and came and drew near to meet David, that David hurried and ran toward the army to meet the Philistine. 49. Then David put his hand in his bag and took out a stone; and he slung it and struck the Philistine in his forehead, so that the stone sank into his forehead, and he fell on his face to the earth. 50. So David prevailed over the Philistine with a sling and a stone, and struck the Philistine and killed him. But there was no sword in the hand of David. 51. Therefore David ran and stood over the Philistine, took his sword and drew it out of its sheath and killed him, and cut off his head with it.
And when the Philistines saw that their champion was dead, they fled.
-1 Samuel 17:48-51