As Jeff said, AWT doesn't support translucency (it supports it, but it's not hardware accelerated)
But you want to draw a rectangle over the image, this can be done with:
g.setColor(new Color(0,0,255,100)); g.fillRect(rx,ry,rw,rh);
The only problem is that not being hardware acceleratred, this drops your performance and (almost) cannot be used in realtime applications.
I use this trick to indicate selections in my tile editor (the tile selected is highlighted in blue).