as Alex explained it to me, all coordinates are based off of the center coordinates. Its X,Y coords when the ship is facing the right of the screen (0 degrees), but for symmetrical ships, you can simply use the inverse, and go with Y,X coordinates without having any perceptible difference. So if the center coords are set to 50,50, then you have coords of something else listed at X,Y in the file, the actual coordinates on the bitmap are 50+X,50+Y
Center coords are taken from the bottom left of the sprite's image file