https://github.com/discourse/discourse/commit/6ae0c42c0156ce3f4861731e4872be026be2eeef
@bartv this PR should fix your current issue with that post. After you update your site if you rebake that post (clicking the Rebuild HTML button) it should switch to using the first image as the topic preview. I’m currently working on another PR to address the blank video thumbnail issue that sometimes occurs.