If you're making game sprites, it saves a lot of work if you make the

lightsource come from the top, because let's say you chose the light was

purely coming from the left, then the facing left sprite would be illuminated
whilst the facing right sprite (which is only a MIRROR of the left sprite), has
to be in shadow. Which makes you do more work, and take's up more of
the game's data.

So in short, I recommend you put the lighting at the top.