var async = require("async"),
    Kraken = require("kraken"),
    fs = require("fs");

var kraken = new Kraken({
    api_key: "your-api-key",
    api_secret: "your-api-secret"
});

var yourArray = [
    "/this/is/file/one",
    "/this/is/file/two",
    "/this/is/file/three"
];

async.forEach(yourArray, 10, function (filePath, cb) {
    // here you process your files with concurrency of 10 at a time

    var opts = {
        file: fs.createReadStream(filePath),
        wait: true
    };

    kraken.upload(opts, function (data) {
        if (data.success) {
            console.log('Success. Optimized image URL: %s', data.kraked_url);
        } else {
            console.log('Fail. Error message: %s', data.message);
        }
        
        
        // this is very important. Calling this callback (cb) tells async
        // that this async function is now complete so it can process another one
        
        cb();
    });
}, function (err) {
    if (err) {
        console.log(err);
    }

    // this is a final callback called only once
    // when all files have been processed.
});