Force search for artwork to a specific folder

Greetings, I would like to develop a simple plugin for a Windows server that would enable Plex to search in a specific folder for posters. In _thumbs folder in this case.

C:/Movies
----_thumbs
--------Movie1.jpg
--------Movie2.jpg
----Movie1.mkv
----Movie2.mkv

I haven’t done any Plex development before. Could someone point me in the right direction where to start or some examples I could chew on?

Thank you,