Render an image in base64 using phantom js. Convert the base64 into a raw binary buffer, pass the binary buffer to GraphicsMagic which can write to a writable stream, collect the chunks of data in separate buffers, storing them in an array and then concatenate them all together.

gistfile1.js

// PhantomJS Render to base64 function
page.renderBase64('png', function(err, img) {
    var image = {};
    image.thumb = [];
    final.full = new Buffer(img, 'base64'); // Convert the base64 encoded value into binary.
    var thumbStream = new stream.Stream();  // Create a new writable stream
    thumbStream.writable = true;
    thumbStream.write = function (chunk) {
        image.thumb.push(chunk);            // Take the data written and store it into an array
    };
    thumbStream.end = function () {
        this.emit('close');                 // Cleanup the stream.
    };
    gm(final.full, 'img.jpg')                   // Enter GraphicsMagic
        .resize('200', '200')                   // ... Resize the image to 200x200 pixels
        .stream(function (err, stdout, stderr) {
            if(err) console.log(err);
            if (stderr) console.log(stderr);
            stdout.pipe(thumbStream);           // Read stdout and write it to our writable stream
            thumbStream.on('close', function () {
                final.thumb = Buffer.concat(image.thumb); // When the stream is done, concatenate the images
                callback(final);
                ph.exit();
                });
        });
});