js 鼠标经过改变图片 js没有相对路径的概念

var demo = document.getElementById('demo');
var imgs = demo.firstElementChild;

demo.onmousemove = function () {
    imgs.src = "效果图2.png";
};
demo.onmouseout = function () {
    imgs.src = "Snip20170119_1.png";
};