I did up a movie poster sort of looking piece of art (or piece of shit depending on how you feel) and threw it on RedBubble. Maybe someone will buy it? Probably not, RedBubble is a bit pricey.
Just your luck, there is actually a poster option on Redbubble. Prints and posters, in fact. Personally I think the posters are alright, but the prints give you some really cool options like glossy metal.
Yeah, Redbubble doesn't do that sadly. I wish they did! I would have had a small SS13 logo on the front and on the back nothing but priceless quotes from the game such as "Have a secure day!", and "Shitcurity is harmbattoning me!" and "Honk honk motherfucker!" and "GET DAT FUKKEN DISK" and "FUCK CHEMISTRY"
Oh and to answer your question, it's which side do you want the image printed on. Redbubble lets you, the consumer, choose which side it gets printed on.