Python tkinter GUI - change default icon: Windows vs. Linux

I'm learning Python at the moment... and as part of that I've been looking at creating platform-independent applications.

The first step, I figured, was to get a basic window operating cross-platform.  I did this, no problem.  I did some reading on tkinter, which seems pretty straight forward (apart from being Tkinter in Python 2.x and tkinter in Python 3.x - note the case change - pretty annoying for converting apps from the old version).  So that was all good anyway.