On the Mac with the Cocoa/Aqua UI style, push buttons don't show icons customarily. For this reason we tried to avoid them in PI 1.7.
Starting from version 1.8 of PixInsight, this rule does not apply because we no longer use native platform-dependent UI styles. PixInsight 1.8 uses a universal UI style that looks and behaves exactly the same on all platforms, including Mac OS X, Windows and X11. The only exceptions are a few differences in the default fonts used on Windows and Mac OSX, and the window minimize/close buttons located at the upper left corner on Mac OS X. So you don't need most of those pesky __PI_PLATFORM__ conditionals anymore