Rotate in multitouch does not work properly

Hello:

When I use the code below to rotate a image with multitouch screen I rotate but on a corner. Can anyone tell me how to rotate in the center of the image?

Thank you very much.

import flash.display.StageDisplayState;

import flash.display.StageScaleMode;

import flash.display.StageAlign;

import flash.events.Event;

import flash.events.MouseEvent;

import flash.display.Sprite;

import flash.events.TransformGestureEvent;

import flash.ui.Multitouch;

import flash.ui.MultitouchInputMode;

Multitouch.inputMode = MultitouchInputMode.GESTURE;

/*stage.displayState = StageDisplayState.FULL_SCREEN_INTERACTIVE;

stage.scaleMode = StageScaleMode.NO_SCALE;*/

stage.align = StageAlign.TOP_LEFT;

var con:Sprite = new Sprite();

con.x = stage.stageWidth * .5;

con.y = stage.stageHeight * .5;

addChild(con);

var b:Sprite = Sprite(new f1());  // my image

b.x=-135;

b.y=-400;

b.addEventListener(TransformGestureEvent.GESTURE_ROTATE, onRotate);

con.addChild(b);

function onRotate(e:TransformGestureEvent):void

{

    var b:Sprite = Sprite(e.currentTarget);

    b.rotation +=  e.rotation;

}