특성이미지 제거 CSS 코드 
.attachment-full {
	display:none;
}

.single . inside-article>.featured-image {
display:none;
}

.single-featured-image-header {
display: none;
}


.attachment-post-thumbnail {
        display:none; 
}




첫번째 이미지 특성이미지 설정 

function auto_featured_image() {
global $post;

if (!has_post_thumbnail($post->ID)) {
$attached_image = get_children( "post_parent=$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);
}
}
}
}
// Use it temporary to generate all featured images; 
add_action('the_post', 'auto_featured_image');
// Used for new posts, 
add_action('save_post', 'auto_featured_image');
add_action('draft_to_publish', 'auto_featured_image');
add_action('new_to_publish', 'auto_featured_image');
add_action('pending_to_publish', 'auto_featured_image');
add_action('future_to_publish', 'auto_featured_image');
// 출처: gavick.com