Thanks again.
I have been trying various png images of continents in Inkscape, trying to scale them up to the map size we need. However, even images of individual continents (which are blank outlines, without any topographical features) have so far been too large (possibly far too large) for the module/VASSAL itself to handle.
As a result, I will likely make the map using the single island pieces. There’s no way to make an image that is approximately 60000x25000 pixels small enough to fit, at least not that I can find. Just exporting a big picture in Inkscape takes over 10 minutes.