The posts archive thumbnails are generated based on whatever you select for post type. So if its a video post type, you need to supply a link to the video in Theme General Settings > Media. When you do that, it will automatically put the video at the top of the post as well. You can turn that off in Theme General Settings > Media if you want.
I did that for you in the Foxy Expectations post
http://community.bridgingstars.org/category/videos/
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