Is it possible to use Ps touch to remove the background from jpeg images and export them locally as png with transparent background (reducing the size of the picture)? I would need to use this feature to create more keynotetouch-friendly images.
Yes. That's possible. You can export a PSTouch project as png to the Camera Roll....and there are other channels as well where this is possible, like "Share By E-mail" , "Send to iTunes" etc.
Those options are accesible via the Share menu (see attached screenshot) as part of the PSTouch project organizer