Why are my JButtons not transparent after using the setOpaque() function?

Question

I've set my JButtons to be transparent and I added pictures with transparent background to them, but they refuse to become transparent. Moreover, it works on Windows but does not work on my MacBook.

I tried

for(int i = 0 ; i < champions.size(); i++) {    
    JButton B = new JButton();  
    B.setActionCommand(champions.get(i).getName());
    B.setPreferredSize(new Dimension(140,196));
    B.setBackground(new Color(0.0f,0.0f,0.0f,0.0f));
    B.setOpaque(false);
    B.setContentAreaFilled(false);
    B.setContentAreaFilled(false);
    B.setIcon(new ImageIcon("Images/"+champions.get(i).getName()+ ".png"));
    B.setRolloverEnabled(false);
    buttons.add(B);
    panel2.add(B);
}

So basically, I tried every way of setting it to transparency, but nothing worked!!!