There is a specific microdata scheme for "videoObject"
(http://support.google.com/webmasters/bin/answer.py?hl=en&answer=2413309) which
this add-on can add to your page, which Google can use to display the video
thumbnail next to the search-result. Note that this is optional and can be
disabled in the ACP. Also the videoobject microdata is NOT added for audio-only
embeds or playlists. Google will not always display the thumbnail, this
presumably depends of the relevance of the video to the rest of the page.
(http://support.google.com/webmasters/bin/answer.py?hl=en&answer=2413309) which
this add-on can add to your page, which Google can use to display the video
thumbnail next to the search-result. Note that this is optional and can be
disabled in the ACP. Also the videoobject microdata is NOT added for audio-only
embeds or playlists. Google will not always display the thumbnail, this
presumably depends of the relevance of the video to the rest of the page.