javascript - onmousemove using 2 parameters

I probably got a quite simple question. I generate some images using javascript. when i click on such an image and move the cursor the image should follow until i click again.
That is my solution:

I create the img and connect it with an "onclick" event handler:

img.onclick = prepare_pic_move(img);

This kind of handler starts the "onmousemove" handler:

  function prepare_pic_move(img) {
img.onmousemove = pic_move(ev, img);
  }

That function should finally move the picture:

  function pic_move(ev, img) {
    img.style.left = ev.ClientX;
    img.style.top = ev.ClientY;
  }

It does not work and the firefox bug console says that ev is not defined.

I think the problem is that i have to use two parameters at the pic_move function.