Currently, the jwm code uses some fast but ugly software scaling. XRender can be used to scale X11 pixmaps, with a filter that makes them more beautiful despite being scaled. Perhaps you can write code to use XRender if it is available, but keep the old code so there is a fallback. I know this can be done because I've played with it before. It works well, but the filters get disabled for some reason if you scale anything beyond 300x300, at least on my system. Most icons fall well below this. If you need some code samples or other help, I can provide some.