Nice tool! It’s an often requested feature so eventually I do hope to get around to building this right into Tiled as some kind of “import map from image” feature.
Btw, I’m a bit wary of running binaries (which includes .jars), since there is no way to know what it’ll do. What about publishing the source code?
Other tools that cover this use-case are Map-Tiler, ShoeBox and the scripts covered in this wiki page.
Edit: I recently ran into an Online Tileset Extractor, which is worth a try as well.