gistfile1.js

var parallelResizes = 5;
async.mapSeries(files, function (item, cb) {
    im.identify(source_dir + file, cb);
}, function (err, identifies) {
    var queue = async.queue(function (options, callback) {
        im.resize(options, callback);
    }, parallelResizes);
    identifiers.forEach(function (id) {
      /* create options objects for every identifier-size pair and push each like this: */ queue.push(optionsSize1, optionsSize2,...);
    });
    queue.drain = function () {
        console.log("resizing complete");
    }
});