There are about 3 ways that I know you can do it. First is to have a sailmaker imprint the image in ink. This seems to be the most permanent but most costly option. Second, you can have a sailmaker or graphics supply (that offers exterior vinyl signage) make the image you want and then you, or a sailmaker, can apply it to the sail. This second option is more affordable but I've gone this route a couple times and while it works well, trying to apply a flat vinyl "sign" to a shaped sail isn't as easy as it might appear at first glance.
The third option is the least expensive, you can do it yourself, but you will have to expect to spend hours and hours doing the work. You print out segments of your graphic, tape them up into to a full size template and then outline and color with a permanent Sharpie marker. Below is one I did for the headsail on my Astus. Took a week to do, working about 3 or 4 hours per day. Materials included about 60 Sharpie markers and about that same number of beers. But it looks good and has held up well.

