將下方代碼添加到當前主題的functions.php中

function wpforce_featured ( ) {  
    global $post ; 
    $already_has_thumb = has_post_thumbnail ( $post - > ID ) ;  
    if ( ! $already_has_thumb ) {   
        $attached_image = get_children ( "post_parent=$post->ID&post_type=attachment&post_mime_type=image&numberposts=1" ) ;    
        if ( $attached_image ) {  
                foreach ( $attached_image as $attachment_id = > $attachment ) {      
                set_post_thumbnail ( $post - > ID , $attachment_id ) ;
            }
        }
    }
} //end function 
add_action ( 'the_post' , 'wpforce_featured' ) ;
add_action ( 'save_post' , 'wpforce_featured' ) ;
add_action ( 'draft_to_publish' , 'wpforce_featured' ) ;
add_action ( 'new_to_publish' , 'wpforce_featured' ) ;
add_action ( 'pending_to_publish' , 'wpforce_featured' ) ;
add_action ( 'future_to_publish' , 'wpforce_featured' ) ;

上面的代碼就會設置文章內的第一張圖為特色圖片。但是有時候我們文章裡沒有圖片怎麼辦？

可以用下面的代碼，設置某一個ID的圖片為默認圖片。

function wpforce_featured ( ) {  
    global $post ; 
    $already_has_thumb = has_post_thumbnail ( $post - > ID ) ;  
    if ( ! $already_has_thumb ) {   
        $attached_image = get_children ( "post_parent=$post->ID&post_type=attachment&post_mime_type=image&numberposts=1" ) ;    
        if ( $attached_image ) {  
            foreach ( $attached_image as $attachment_id = > $attachment ) {      
                set_post_thumbnail ( $post - > ID , $attachment_id ) ;
            }
        } else {  
            set_post_thumbnail ( $post - > ID , '8888' ) ;
        }
    }
} //end function 
add_action ( 'the_post' , 'wpforce_featured' ) ;
add_action ( 'save_post' , 'wpforce_featured' ) ;
add_action ( 'draft_to_publish' , 'wpforce_featured' ) ;
add_action ( 'new_to_publish' , 'wpforce_featured' ) ;
add_action ( 'pending_to_publish' , 'wpforce_featured' ) ;
add_action ( 'future_to_publish' , 'wpforce_featured' ) ;

上面代碼裡的$post->ID, ‘8888’這個8888就是圖片ID，自己在網站媒體庫找一張圖改一下ID吧。