I've been having this problem for awhile. I store all of my posters locally so that my library is complete should it ever need to be regenerated, I won't need to go in and assign posters or edit metadata.
I often assign posters for the "Specials" or "Season 00" that are not necessarily available on tvdb. I label them as:
\Gilligan's Island\Season 00\season00.jpg
This does not work. I've tried several different ways of naming it, but I've never gotten it to work. It works just fine when I use the same structure for a regular season.
\Gilligan's Island\Season 01\season01.jpg
My Agents are as follows:
Local Media Assets (TV)
Plex Theme Music
The TVDB
Any thoughts or ideas are appreciated.
Mike