how to make nsfw post with thumbanil

by

get_post_meta is your friend:

if ( has_post_thumbnail() )  {
    if ( 'yes' == get_post_meta( $post->ID, 'NFSWEG', true ) ) {
        echo '<img src="https://wordpress.stackexchange.com/questions/82422/YOUR-IMAGE-HERE">';
    } else {
        the_post_thumbnail(
            array( 400, 400, true ), 
            array( "class" => "aligncenter featured_image" )
            ); 
    }
}