Question

Is they a way of adding a watermark to a JTextArea?

Answer 1

I suspect that you’d need to subclass JTextArea and override the paintComponent() method, drawing your background image first and calling super.paintComponent() to render the text:

public void paintComponent (Graphics g) {
    g.drawImage(watermark, 0, 0, this);
    super.paintComponent(g);
}

edit: as pointed out by camickr, a JTextArea is opaque, so your subclass will need to change this by calling setOpaque(false).