"La définition de l'image est bien meilleure."
Translation:The picture's definition is much better.
While the two terms may go hand in hand, I would stick with definition. Think of it in context of television. The definition is much better on my HDTV (high-definition TV) than on my old one. Image sharpness would be "netteté de l'image" (I believe. Correct me if I'm wrong someone).