I found a link to a page but it's 404. https://skybox.xyz/support/How-to-add-custom-thumbnail-to-your-video/
Bijon Hello,
To create a custom thumbnail, just put the image (JPG, JPEG, and PNG format) and video file in the same folder with the same file name (except for the file extension).
For example: Directory: /Movies movie_name.mp4 movie_name.jpg
Thanks for the reply. I can't get it to work, though. They only show as separate images in the folder. Maybe there's an option I'm missing?
Never mind, after a reboot they work!