Hrm.... when I add a movie as a single folder and then do "rescrape movie" it does not resize the fan art. I mean the button with the arrow on it. This includes when I have deleted the movie-fanart.jpg files.
However, after some fiddling about I tried hitting the button with the + on it. That does appear to resize the fanart.
So it seems, for movies, it's just when you re-scrape that it doesn't resize the fanart.
At least that has been my experience.