ok risolto così:
codice:
    public void resizeImage(File inputImage, String dir, int width, int height) throws IOException {
        Image image = ImageIO.read(inputImage);
        Image newImage = image.getScaledInstance(width, height, Image.SCALE_SMOOTH);
        BufferedImage buf = new BufferedImage(newImage.getWidth(null), newImage.getHeight(null), BufferedImage.TYPE_INT_RGB);
        Graphics2D g2 = buf.createGraphics();
        g2.drawImage(newImage, 0, 0, null);
        g2.dispose();
        String ext = FilenameUtils.getExtension(inputImage.getName());
        ImageIO.write(buf, ext, new File(dir + inputImage.getName()));
    }
grazie per la dritta!