您可以尝试以下操作：

/**
 * Get the featured image by post title (Simple version)
 * 
 * @see    http://wordpress.stackexchange.com/a/158344/26350
 * @param  string $title   Post title
 * @param  mixed  $size    Featured image size
 */

function get_featured_image_by_post_title_wpse_simple( $title = \'\', $size = \'thumbnail\' )
{
    $obj  =  get_page_by_title( $title, OBJECT, \'post\' );
    return ( is_object ( $obj ) ) ? get_the_post_thumbnail( $obj->ID, $size ) : \'\';
}

/**
 * Get the featured image by post title (Extended version)
 *
 * @see    http://wordpress.stackexchange.com/a/158344/26350
 * @param  string $title     Post title
 * @param  mixed  $size      Featured image size
 * @param  string $post_type Post type
 * @param  string $default   Default image url
 * @return string $html      Featured image HTML
 */

function get_featured_image_by_post_title_wpse_ext( $title = \'\', $size = \'thumbnail\', $post_type = \'post\', $default = \'\' )
{
    // Search by post title:
    $obj  = get_page_by_title( $title, OBJECT, $post_type );

    // Featured image:
    if( is_object( $obj ) && has_post_thumbnail( $obj->ID ) )
        $html = get_the_post_thumbnail( $obj->ID, $size );
    else     
        $html = sprintf( \'<img src="%s" alt="">\', esc_url( $default ) );

    return $html;
}

Usage examples:

 // Simple:
 echo get_featured_image_by_post_title_wpse_simple( \'Hello World!\', \'large\' );

 // Extended:
 echo get_featured_image_by_post_title_wpse_ext( \'My Car\', \'full\', \'post\', \'/car.jpg\' );