The plugin uses TheTVDb API, and there are apparently some limitations. More about that
here. It's unclear to me what the potential is for improvement, like getting images.
I'm not sure what you mean by, "I see no separate entry in the .pvd database." Multiple posters, covers and screen shots can be saved in series and episode records. Currently, there is no ability to record any information about seasons. This has been requested, and may be added in the future.
Generally, I like the art available at TheTVDb. I often get a series poster there, and sometimes attach some fan art to the series record. (A large full cast photo is a handy visual reference.) It would be nice to be able to attach these things at a season level. For episodes, the images are just other users screen shots. While downloading these would be more efficient, I'm resigned to a work flow where I manually copy one screen shot from an automatically generated set, saving this as a poster and then deleting the screenshots. Works okay in the summer, but with a new season approaching, I think I'm going to be wishing for a less labour-intensive approach.
