insertImg = function renderItemInsertImg() {
	            var simg = ' src="' + item.content + '"';
	            // auto scale to full screen
	            if (item.height / item.width > self.ratio) {
	                simg += ' height="100%"';
	            } else {
	                simg += ' width="100%"';
	            }
	            if (self.isOverspread) {
	                el.style.cssText += 'background-image:url(' + item.content + ');background-repeat:no-repeat;background-position:50% 50%;background-size:cover';
	                simg += ' style="display:block;opacity:0;height:100%;width:100%;"';
	            }
	            // for right button, save picture
	            el.innerHTML = '<img' + simg + ' />';
	        }
render single item html by idx
@param {HTMLElement} el ..
@param {Number} dataIndex  ..
@private