eski, I assume when you say 'make the ball roll realistic', you are talking about just the texture? (which should be another sprite attached to the physics ball).
You should set the animation speed to:
someConstant * distance(0,0,velocityX,velocityY)
where someConstant is a number. You will have to play with it until it looks natural.
You will also need to set the angle of the image to
angle(0,0,velocityX, velocityY)
(That is assuming the rolling ball animation is rolling to the right).
Hope that helps! Be sure to ask me with sqiddster if that doesn't work as I often lose topics like this :P