batch.js

var fs = require('fs'),
    request = require('request');

var parts = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
var checked = [];
var use_thumbs = true;
var max = 20000;

function download(uri, filename, callback){
  request.head(uri, function(err, res, body){
    if( Number(res.headers['content-length']) === 503 ) {
        return getImage();
    }
    console.log('content-type:', res.headers['content-type']);
    console.log('content-length:', res.headers['content-length']);

    request(uri).pipe(fs.createWriteStream(filename)).on('close', callback);
  });
}

function makeId() {
    var id = '';
    for (var i = 0; i < 5; i++) {
        id += parts.charAt(Math.floor(Math.random() * parts.length));
    }
    if (checked.indexOf(id) === -1) {
        checked.push(id);
        return id + (use_thumbs ? 's' : ''); // the s gives us the thumbnail. 
    }
    return makeId();
}

function getImage() {
    var id = makeId();
    download('http://i.imgur.com/' + id + '.png', Date.now() + '.png', function() {
        console.log('done');
        if( max-- >= 0 ) {
            setTimeout(getImage, 10);
        }
    });
}

getImage();