Image flash up before effects starts

Hi, I'm trying to transition in my image but it flash up just before the animation starts, this is the code I'm using:

var holder:Sprite = new Sprite();
addChild(holder);
var sports:Bitmap = new FACE_STUDIO_HIRES()
holder.addChild(sports)
var _effect:FlashEff2Flex = new FlashEff2Flex();
addChild(_effect);
var showEffect:FESSquareExplode = new FESSquareExplode();
showEffect.squareWidth = 20;
showEffect.squareHeight = 20;
_effect.target = holder;
_effect.showTransition = showEffect

Where do you use that code in Flash Builder or in Adobe Flash CS? Also are you sure that you don't receive an error? Or what exactly flashes?