That girl had a different pose. Here's a youtube clip of her performance. You'll notice that her butt is nearly above her head and therefore her center of mass is pretty much above her arms. That's why she can keep her balance in this position. But in the fanart above, Jason's center of mass is probably somewhere behind his arms. So if he'd try to hold this pose for longer than a moment he'd normally loose balance and fall down. A heavier bow would shift the center to the front - but I don't think his toes/feet are strong enough to hold a 50 kg bow like that.