Selecting image for embedded youtube video
Sadly you cant if its not your video. This is chosen either by youtube or if your a youtube partner you can upload your own image (I think they have a tool nowadays which lets regular youtube users choose their thumbnail for their video).

If you visit this page:

https://developers.google.com/youtube/2.0/developers_guide_jsonc

You will see down the page there is a default/hqdefault thumbnail that comes with every video feed.