newFixedThreadPool slower than single threaded program

Here's a quick rundown of my predicament. I am leaing multi-threading and therefore decided to make a thumbnail creator program to process around 300 images. At first, I did all the thumbnail creation on a single thread and that took around 99 seconds. So I figured that using newFixedThreadPool() would make the program faster. I have heard that using Executors.newFixedThreadPool() is less expensive than manually creating threads. So I did ExecutorService executor = Executors.newFixedThreadPool(4);

4 being the number of cores on my machine.

The strange thing is that doing this made the program slower. When I used one thread in the newFixedThreadPool, the tasks were completed in around 118 seconds. When I used 4 threads, the tasks were completed in around 99 seconds. A few milliseconds slower than the initial single threaded operation. When I used 10 threads, the time taken was around 110 seconds.

I have absolutely no idea on how to make the program faster. I was thinking that using 4 threads is optimal and it would allow for the work to be done in around a quarter of the time the single threaded operation took.

Does anyone have any suggestions on what can be done? Here is my (very ugly code)

public class Thumbnails {
    protected static BufferedImage image = null;
    protected static BufferedImage outBM;
    protected static Graphics2D g2d;

    public static void main(String[] args) {

        long start = System.currentTimeMillis();
        File f = new File("/home/njengah/Media/Images");

        File[] myPics = f.listFiles();// list of all images folder

        ExecutorService executor = Executors.newFixedThreadPool(4);

        for (int i = 0; i < myPics.length; i++) {
            executor.submit(new Thumbnails().new ImgProcessor(myPics[i]));

        }
        System.out.println(" all tasks submitted ");

        executor.shutdown();


        try {
            executor.awaitTermination(1, TimeUnit.HOURS);
        } catch (InterruptedException e1) {
            e1.printStackTrace();
        }

        System.out.println(" image processing completed.");

        long stop = System.currentTimeMillis();

        System.out.println("time taken to process all images: "+(stop - start));

    }

    // this handles the processing of a single image
    protected static void singleImageProcessor(File onePic) throws IOException {
        image = ImageIO.read(onePic);

        outBM = new BufferedImage((int) (image.getWidth() * 0.1), (int) (image.getHeight() * 0.1), image.getType());

        g2d = outBM.createGraphics();

        g2d.drawImage(image, 0, 0, (int) (image.getWidth() * 0.1), (int) (image.getHeight() * 0.1), null);
        g2d.dispose();

        ImageIO.write(outBM, onePic.getName().split("\.")[1], new File("/home/njengah/Downloads/resized/" + onePic.getName()));
    }

    private class ImgProcessor implements Ruable {

        File onePic;

         public ImgProcessor(File onePic) {
             this.onePic = onePic;

        }

        @Override
        public void run() {
            // this runs one task
            System.out.println("starting");
            try {
                singleImageProcessor(onePic); // single task

            } catch (Exception e) {
                e.printStackTrace();
            } // to make it shut up

            System.out.println("finished");
        }

    }

}