This relates to the Other Video library type at least, it might apply to the others as well but I have not tested it. When you delete a video it deletes the video file and associated vtt file but it does not also remove the poster image file if there is one resulting in orphaned image files. It would be nice if those files could also be deleted at the same time.
Example
Finish watching a video and then select delete the following happens.
MySampleVideo.en.vtt - deleted
MySampleVideo.jpg - left behind
MySampleVideo.webm - deleted
Thanks for your suggestion. There’s already an existing thread discussing an option to delete the entire content of a movie folder when deleting the movie from within Plex (incl. posters). I suggest you comment/vote in that thread in order to help us avoid distracting or cannibalizing votes. Unless I’m missing some unique aspect of your suggestion I’ll close this thread as a duplicate.