I've always thought it was the back. But logically then we would be looking down at him. Since we cant see the city, only the sky, we have to be looking up at him.
Also.. Does Batman wear high heels here?! He's a ninja, they should know better than anyone that flat, thin soles are quieter.
The siluette would be the same no matter if its the left or right arm, assuming this is a faraway shot and has barely any perspective. Think of Wile E. Coyote when he leaves a hole in something. It wouldnt matter if he hit the wall with his front first or back first, it would still be the same contour.