Nope, it doesn’t work that way. Plex downloads the posters in the order given by the agent. The first one, which is usually the most popular, is used as the default poster by PMS. If you do nothing to your collection, this will never change. If you Refresh your movie, then yes, it may pull new data and if the poster from the source has changed, so will the one used by PMS. If you however, manually edit your movie and select a different poster, that one will be chosen and locked. This locked poster will not change, unless #1 in my comment above applies.
