I'm sure there is a logical reason for why the stamp is nearly always on the left as the Japanese have a purpose for everything they do. I assume since the majority of the planet's population is righty, they stamp where the majority will see it, but its probably not that simple.
As for why the photos show the Kanji on the left... I know it sounds like a smartass response, but... its because the Kanji IS on the left.
A picture of the backside clearly doesnt sell, well.