Thumbnail selected in the Shortcode dialog will create responsive custom thumbnail with multiple images sizes from your WP Media Gallery. Browsers pick the right size depending on device, pixel density … (srcset)
Manually entered URL to be used as thumbnail. This method has no GUI, does not support multiple image sizes and is not recommended, but there to keep backwards compatibility.
If no thumbnail is set the ARVE Pro Addon is able to get Thumbnails automatically from the following providers that support this.
Collegehumor (Only very small 175×98 thumbnails)
Using the “hide title” option for this video as it already has the title visible on the thumbnail.