Could you just have the big stickers made up and stick them on your existing sails? At least I'm guessing that they are stickers. I've seen a couple boats that have sponsor logos on the sails, and they appear to stick to the sail in the same fashion that sail numbers do.
That's how most sailors apply their sponsor logos to the sails, with vinyl stick-ons.
But Photosails actually impregnates the sail material somehow with a photo that covers the whole sail. I don't know how they do it, but I picture it as sort of like silk-screening a T-shirt. It is a patented process, and they don't really explain it on their web site.
