If the size of control button images of current theme is smaller, the GTK LAF resizes the same to expected size in JInternalFrame. This makes the UI ugly.

Instead of trying to resize, it will be better to place the image at the center of the button, so that atleast the UI looks good. I do agree, this will affect the feel, since the button will be active outside this image as well. Probably the contains(int x, int y) method of Component class can be overriden so as to give appropriate feel for the button.

It will be great if the control images of JInternalFrame are placed at the same location as in frame title bar.

Refer to attached screenshot from SwingSet2 in GTK LAF

Solaris 9.0 Sparc GNOME 2.0
Widget Theme : Default
Window Frames: AgingGorilla

Also its noted that, when "Window Frames" theme is changed, the JInternalFrame's control buttons are not changed. These too have to reflect the changes made to Window Frames's theme.