I've searched for this issue and can see it's come up sporadically but there doesn't seem to be a fix for it.
Using Plex as a DLNA server to watch local files works like a charm in every way but one: the thumbnails used for the videos. I set my own 'poster' image, but after I've watched the video and return to the folder, the thumbnail is stretched and no longer optimized. This only seems to happen when I apply 'side by side' view in DeoVR (which is essential, obviously).
It's to the point now where Plex is no longer a viable option unless there is a fix for this.