how to set JFrame background transparent but JPanel or JLabel Background opaque?

As per assignment, we have to create a image viewer just like Picasas one. picture in the middle, translucent black background and changing images with left/right buttons.

i can display an image set it to undercoated, set it to translucent frame but along with frame the the picture becomes translucent. what am i doing wrong.

Dimension dim = Toolkit.getDefaultToolkit().getScreenSize();   

JFrame f1 = new JFrame("ShowImage");
f1.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

f1.setSize(dim);

f1.setUndecorated(true);
f1.setOpacity(0.5f);
ShowImage panel = new ShowImage();
panel.setBackground(Color.black);

f1.setContentPane(panel); 
f1.setVisible(true);  

i tried

si.setOpaque();   
si.setBackground(Color.black);
si.setForeground(Color.red);

none worked

when i took a boolean and tested

si.isDisplayable();
si.isVisible();
si.isShowing();

only is visible returns true, rest are false, are these any contributing factor?