Thumbnails not added

I was just trying to add some videos with Spider Video Player, but every time I add a youtube video and try to add a thumbnail (no matter whether from the library, a link or an immediate upload), it does not show up in the video player page at all. Yes, I clicked “Insert into post”.

Any idea what the problem might be? This is the only issue that prevents me from buying a full version so far…

http://wordpress.org/extend/plugins/player/