[cihexe]

сравни это :

                                  var picture:Picture = new Picture(vars["foto_url" + i]);
			addChild(picture);
			pictures.push(picture);
			picture.posX = 100;
			picture.posY = 100;
			picture.addEventListener(MouseEvent.MOUSE_OVER, mouseOverBox);

и это:

var selectedBox:MovieClip = MovieClip(evt.target);

попробуй подставить:

var selectedBox:Picture = Picture(evt.target);