var app = require('express')();
var http = require('http').Server(app);
var io = require('socket.io')(http);

app.get ('/', function(req, res){
    res.sendFile(__dirname + '/index.html');
});
http.listen(80, function(){
    console.log('listening on *:80');
});

<img src="img/myimage.png">

myApp - folder
    index.js
    index.html
    img - folder
        myimage.png

<img src="/img/myimage.png">
<img src="./img/myimage.png">

<img src="myimage.png">

var app = require('express')();

var express = require('express');
var app = express();

app.use(express.static(__dirname + '/img/'));

const express = require("express");
const app = express();

app.use('/img', express.static(__dirname + '/Images'));