Video preview image quality

You can modify the thumbnail preview used with the filter rocket_youtube_thumbnail_resolution.

The default value is hqdefault, and the accepted values are: default, mqdefault, sddefault, hqdefault, maxresdefault

so you could do this:

add_filter( 'rocket_youtube_thumbnail_resolution', function( $thumbnail_resolution ) {
    return maxresdefault;
} );

Sorry about that, the correct filter name is rocket_lazyload_youtube_thumbnail_resolution

Also note that the maxresdefault thumbnail is not always available from Youtube