Video best practices – Search Console Help

Provide a high-quality thumbnail for your videoTo be eligible to appear in Google Video Search results, a video must have a thumbnail image that can be shown in search results.You can provide (or enable) a thumbnail in several ways:If using the HTML tag, specify the poster attribute.In a video sitemap, specify In structured data, specify VideoObject.thumbnailUrlProvide a video in a crawlable format and we can generate a thumbnail for you.Preferred formats: JPG, PNGSize: From 160×90 to 1920×1080 pixelsLocation: The preview thumbnail must be accessible by Googlebot (that is, not blocked by robots.txt or a login requirement).

來源: Video best practices – Search Console Help