If I remember rightly there is a Program/script combo which KGC created for Tileset Extension which I think can solve your problem. Unfortunately I do not run Windows XP so am not positive on it's usage, but you can find the download for it
here and Celianna wrote a little about how it works too, however this is only about the useage for Parallax mapping, but I am sure it can work in the same way for standard mapping
Hope that helps