Hi,
it is fairly simple. You just have to use the AlphaComposite:
// g is the Graphics object
Graphics2D g2d = (Graphics2D)g;
//0.5 to display at 50% transparency
g2d.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, (float) 0.5));
g2d.drawImage(...);
Cheers.
Vince