loadImg = function (index) {
	                var item = data[index];
	                if (self._itemType(item) === 'pic' && !item.load) {
	                    var preloadImg = new Image();
	                    preloadImg.src = item.content;
	                    preloadImg.onload = function () {
	                        item.width = preloadImg.width;
	                        item.height = preloadImg.height;
	                        item.load = 2;
	                    };
	                    preloadImg.onerror = function () {
	                        item.load = -1;
	                    }
	                    item.load = 1;
	                }
	            }
Preload img when slideChange
From current index +2, -2 scene
@param {Number} dataIndex means which image will be load
@private