Recompressing pictures loose much quality, and yours are not compressed enough so we would have to recompress them again, so again some losses.
I think that it is not worth modifying such pictures, better wait that someone updates these from the DVD.
And the current pictures do not come from movie theater cam or something like that, those made by no_car come from a DVD-rip divx (though they are a little too yellow), and same for mine (maybe a little dark, but I do not know why I have that for some movies).
Those that you modified are too bright anyway, even on my dark monitor...