No, Serviio doesn't store posters in the folder - it stores all data, including posters, in its own database. The poster in your folder must have been created by something else, or commonly, downloaded with the movie.
To use online metadata sources, in Serviio Console:
- On the Library tab, ensure that Metadata is ticked (under the M symbol) for your shared folder. Press Save if you change anything;
- On the Metadata tab (Video), ensure top two checkboxes are ticked, dropdown box is set to "Online metadata sources" and "Retrieve DVD Cover Image" is checked. Press Save if you change anything.
For each movie in your library, ensure that the filename complies with the schema (ie. name the file as "movie name (year).xxx" - for example "Toy Story 1 (1995).mkv").
Then go back into Serviio Console and force refresh your library (on the Library tab, press Force Refresh).