You'd have to edit your GRF files for doing that. Just navigate through the GRF until you find the files, then edit them yourself (or, if not available in your GRF, make your own files then add them to your GRF) and, if you're having a live pserver, launch the changes on a patch.
Sadly, I can't help you much more on this since I don't know where these sprites are.
Since this is more of a Client-side request, II hope that you won't minf if I move this topic to the appropriate forum.