Like another poster stated, easiest way is to custom paint the sail and outline the sails (all sails if medium/large) and save it. Then use the converter to get the template. Sails are still 256x256 as far as I can tell (at least the Ark template program is limited to that, and I did get my medium speed sail to work). The reason why it magnifies the corner of your image, is because you cannot fill the whole template with a 256x256 picture. For a medium speed sail, the top left is the bigger bottom sail, and the top right is the top sail. So your image has to be smaller than the 256x256.
At work atm, so can't post a picture yet.
I lost my first Schooner right after SotD were added into the game. No wind + 3 ships on me after logging back in after the patch. I've lost 2 sloops zoning into another area and spawning literally on top of a SotD, again no wind to sail away (smallest wind arrow possible every time). I still attempt to sail as I need to find the resources to be able to build cannon (Island has silver, requires gems to make ignots, no gems on island) so am unable to defend/attack SotD anyway. Catch 22