AFAIK Moodle multimedia filter doesn't allow adding posters to a video, but a user could create the video himself and add a poster to it. I think we should try to download those posters like we do with content images.

Right now the posters aren't seen at all, since the token isn't added to the URL.

I performed some tests and the behaviour of manually added videos seem to be a bit inconsistent throughout Moodle. For example, I added a video to a course topic and I couldn't see it in the app, the WS filtered it. In a book it worked fine.