I did not find the following function how to use it, do you have a more detailed documentation?
“Poster (preview) images can now be used with both local and remote media files. Local poster images can be in .jpg, .png or even .gif formats, but remote poster images are only supported in .jpg.”

by k2 2.7

Simply upload an image in the specified formats with the exact filename as the video in the same folder (as the video).

Thanks, that is, there is no way to use preview images on YouTube videos : (

Since videos are served through YouTube, YouTube serves the preview as well.