How to combine multiple PNGs into one big PNG file?

Create a large image which you will write to. Work out its dimensions based on how many rows and columns you want.

    BufferedImage result = new BufferedImage(
                               width, height, //work these out
                               BufferedImage.TYPE_INT_RGB);
    Graphics g = result.getGraphics();

Now loop through your images and draw them:

    for(String image : images){
        BufferedImage bi = ImageIO.read(new File(image));
        g.drawImage(bi, x, y, null);
        x += 256;
        if(x > result.getWidth()){
            x = 0;
            y += bi.getHeight();
        }
    }

Finally write it out to file:

    ImageIO.write(result,"png",new File("result.png"));