Looks like a ugly Hack, but works for me.
JComponent title = ((BasicInternalFrameUI)myInternalFrame.getUI()).getNorthPane();
for (int i = 0; i < title.getComponentCount(); i++) {
JComponent component = (JComponent)title.getComponent(i);
if(component instanceof JButton) {
JButton button = ((JButton)component);
if(button.getName() == null) continue;
if(button.getName().endsWith("closeButton")) {
button.setIcon(myIcon);
button.setSelectedIcon(myIcon);
button.setPressedIcon(myIcon);
}
if(button.getName().endsWith("maximizeButton")) {
...
}
if(button.getName().endsWith("iconifyButton")) {
...
}
}
}