server.js

const fs = require('fs'),
	  path = require('path'),
	  Twit = require('twit'),
	  config = require(path.join(__dirname, 'config.js'));

const T = new Twit(config.twitter);

const randomFromArray = (images) => {
	return images[Math.floor(Math.random() * images.length)];
}

const uploadRandomImage = (images) => {
	console.log('Opening an image...');
	const imagePath = path.join(__dirname, '/images/' + randomFromArray(images)),
	b64content = fs.readFileSync(imagePath, {encoding: 'base64'});
	
	console.log('Uploading an image...');
	
	T.post('media/upload', {media_data: b64content}, (err, data, response) => {
		if (err){
			console.log('ERROR:', err);
		} else {
			console.log('Image uploaded!');
			console.log('Now tweeting it...');
			
			T.post('statuses/update', {
				/* You can include text with your image as well. */            
				// status: 'New picture!', 
				/* Or you can pick random text from an array. */            
				status: randomFromArray([
					'New picture!',
					'Check this out!'
				]),
				media_ids: new Array(data.media_id_string)
			},
			(err, data, response) => {
				if (err){
					console.log('ERROR:', err);
				} else {
					console.log('Posted an image!');
				}
			});
		}
	});
}


fs.readdir(__dirname + '/images', (err, files) => {
	if (err){
		console.log(err);
	} else {
		let images = [];
		files.forEach((f) => {
			images.push(f);
		});
		
		/*
		You have two options here. Either you will keep your bot running, and upload images using setInterval (see below; 10000 means '10 milliseconds', or 10 seconds), --
		*/
		setInterval(()=> {
			uploadRandomImage(images);
		}, 10000);
		
		/*
		Or you could use cron (code.tutsplus.com/tutorials/scheduling-tasks-with-cron-jobs--net-8800), in which case you just need:
		*/
		
		// uploadRandomImage(images);
	}
});