gistfile1.js

var Promise = require('bluebird')
var mkdirp = Promise.promisify(require('mkdirp'))
var fs = Promise.promisifyAll(require('fs'))
var request = require('request')

var resolutions = {
    1: [  1,  1 ],
    2: [  2,  2 ],
    3: [  5,  4 ],
    4: [ 10,  9 ],
    5: [ 23, 19 ],
    6: [ 47, 39 ]
}

var prfx = 'http://promo.na.leagueoflegends.com/sru-map-assets/'

Promise.map(Object.keys(resolutions), function(idx) {
    return mkdirp('map/' + idx)
                .then(downloadMap.bind(this, idx))
                .then(function(x) {
                    console.log('...done')
                    return x
                })
}, { concurrency: 1 }).then(function() {
    console.log('all downloads complete')
})

function pad(n) {
    return ('000' + n).substr(-2)
}

function downloadMap(idx) {
    console.log('downloading resolution: ', idx)
    var res = resolutions[idx];
    var urls = []
    for(var x=0;x<=res[0];x++) {
        for(var y=0;y<=res[1];y++) {
            var url = prfx + idx + '/' + x + '/' + y + '.png';
            var path = 'map/' + idx + '/' + pad(x) + 'x' + pad(y) + '.png';
            urls.push([ url, path ] )
        }
    }
    return Promise.map(urls, function(url) {
        return new Promise(function(resolve, reject) {
            request(url[0])
                .pipe(
                    fs.createWriteStream(url[1])
                ).on('finish', resolve)
        })
    }, { concurrency: 100 })
}