How to stop or replace autogenerated images for library type "Other Videos"

Server Version#: 1.41.8.9834
Player Version#: 10.30.2.3548

I’m trying to find a solution to an annoying issue with the image that Plex creates for each video, for he library type “Other Videos”. Specifically this is the images displayed for each video in the folder view of video files, as well as the full screen images shown as the backdrop for the GUI, when a video has been selected. I use the Other VIdeos library type for a large collection of motorsport videos, and the issue is that Plex almost invariably creates these images from a time around the end of a race, often giving away the result.

I’d like to know if there is a way to either stop these images being generated, or to replace them with an image of my own. I’ve tried dropping a JPG file, with the same name as one of the video files, into the folder containing the video as a test (then rescanning the library), but Plex still displays the generated image instead.