Add a thumbnail - like a personal photo or company logo - to the site header by
params.thumbnail in your site’s configuration file to the path to the
thumbnail image in the assets folder. Give it a rounded edge by
params.thumbnail_radius to a valid CSS
border-radius value. Images are scaled down to a maximum height of 128
pixels, approximately the height of the header with a site title and tagline.
Midnight also adds a line of metadata to every page on the site defining that image as the site’s thumbnail. This tells search engines and other sites to use that image in URL previews.