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.
Excellent point. I was thinking about something similar today. I wanted a scene where the light was reflecting off the lake, but the lake is to the east and the time is late afternoon. No light reflecting off the lake when the sun sets in the west!