File file = new File("F:/firefox.png");

File destFile = new File("F:/pic/" + System.currentTimeMillis() + ".png");

BufferedImage image = ImageIO.read(file);

BufferedImage destImage = new BufferedImage(image.getWidth(), image.getHeight(), BufferedImage.TYPE_INT_ARGB);

ColorConvertOp cco = new ColorConvertOp(ColorSpace.getInstance(ColorSpace.CS_GRAY), null);

cco.filter(image, destImage);

ImageIO.write(destImage, "png", destFile);