Display.update();
GL11.glReadBuffer(GL11.GL_FRONT);
...

public static void takeScreenShot()
   {
      //Display.update();
      GL11.glReadBuffer(GL11.GL_FRONT);
      int width = Display.getWidth();
      int height= Display.getHeight();
      int bpp = 3; // Assuming a 32-bit display with a byte each for red, green, blue, and alpha.
      ByteBuffer buffer = BufferUtils.createByteBuffer(width * height * bpp);
      GL11.glReadPixels(0, 0, width, height, GL11.GL_RGB, GL11.GL_UNSIGNED_BYTE, buffer );
      
      File file = new File("media/screenshots/" + String.valueOf(System.currentTimeMillis()) + ".png"); // The file to save to.
      String format = "PNG"; // Example: "PNG" or "JPG"
      BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);

      for(int x = 0; x < width; x++)
         for(int y = 0; y < height; y++)
         {
            int i = (x + (width * y)) * bpp;
            int r = buffer.get(i) & 0xFF;
            int g = buffer.get(i + 1) & 0xFF;
            int b = buffer.get(i + 2) & 0xFF;
            image.setRGB(x, height - (y + 1), (0xFF << 24) | (r << 16) | (g << 8) | b);
         }

      try {
         ImageIO.write(image, format, file);
      } catch (IOException e) { e.printStackTrace(); }
   }

game loop
    render game
    if screenshot requested
        glReadBuffer(GL_FRONT) <-- wouldn't GL_BACK make more sense?
        glReadPixels etc
    Display.update()