Most people wouldn't have a clue how to do this (myself included).
Just want to make clear that this is not a rule of the competition.
Of course not. I just thought I'd mention it for anyone that's using third-party graphics. It gives a feeling of professionalism that most users wouldn't even notice. If your graphics are reasonably symmetrical, all you do is flip the picture horizontally in your favourite graphics editor. Otherwise, you just switch the highlight and shadow colours. It's a little bit tedious, but very easy when using pixelated graphics.