Resizing images in mc from center point

1

hi guys,
I have an image that loads into an emty mc. When mouse over a button, i want the image to increase in size from the center. I am using a basic script that resizes the mc.


this.onEnterFrame =function(){
        this._width += (w-this._width)/speed;
        this._height += (h-this._height)/speed;
        this._alpha -= step;
        trace(this._width);
        if (Math.abs(this._width)>w and this._alpha<=0) {
            delete this.onEnterFrame;
        }
}

However the image resizes from the top left corner rather than resize from the center.

How do i acheive it so it resizes from the center?

Thanks

Daniel