onEventHandler question

Hi,
i have produced a code for an online gallery which loads an image into a movie clip called frame_mc. on mouse click the loader big_pic loads a requested photo and then displays it on frame_mc (remove/add child etc).

function thumb2click(event:MouseEvent):void {
big_pic.load(img2Req);
frame_mc.removeChildAt(1);
frame_mc.addChild(big_pic);
caption_txt.text = caption2;
caption_txt2.text = caption2_2;
}

function thumb3click(event:MouseEvent):void {
big_pic.load(img3Req);
frame_mc.removeChildAt(1);
frame_mc.addChild(big_pic);
caption_txt.text = caption3;
caption_txt2.text = caption2_3;
}

Now im trying to add a transition effect to the photo change, such that when loading of big_pic is finished, the transition animation is displayed. how would i write this event handler and wat commands do i need?