<小时>Thumbnail from post in child category:

这样，您可以从属于其中一个子类别的随机帖子中检索“类别缩略图”

我已经对代码进行了注释，以使您能够理解这是如何完成的，此外，它应该是非常不言自明的。

/**
 * wpse135208_cat_thumb_from_random_child.
 *
 * Get the thumbnail from a random post belonging to one of the child categories.
 *
 * @version 0.1
 *
 * @link https://wordpress.stackexchange.com/q/135208/22534
 *
 * @param integer $c_cat (default: \'\')
 * @param string/array $size (default: \'post-thumbnail\')
 * @param string/array $attr (default: null)
 *
 * @return string
 */
function wpse135208_cat_thumb_from_random_child( $c_cat = \'\', $size = \'post-thumbnail\', $attr = null ) {
    // do nothing if $c_cat is empty
    if( empty($c_cat) ) return;
    // get_terms is used because we only need ids
    $taxonomies = array(
        \'category\'
        );
    $args = array(
        \'child_of\' => $c_cat,
        \'fields\' => \'ids\'
        );
    // returns an array of ids
    $child_cats = get_terms( $taxonomies, $args );
    // use this for debugging
    //echo \'<pre>\'; print_r($child_cats); echo \'</pre>\';

    $args = array(
        // we use numberposts instead of post_per_page,
        // because if the pre_get_posts filter is used,
        // it can make a difference in this case 
        // we only want one post
        \'numberposts\' => 1,
        // but we randomize this
        \'orderby\' => \'rand\',
        \'category__in\' => $child_cats,
        \'fields\' => \'ids\',
        // make sure only posts with featured image are considered
        \'meta_query\' => array(
            array(
                \'key\' => \'_thumbnail_id\',
                \'compare\' => \'EXISTS\'
            )
        )
        );
    // returns an array containing one post id
    $ct_p_id = get_posts( $args );
    // use this for debugging
    //echo \'<pre>\'; print_r($ct_p_id); echo \'</pre>\';

    // use this for debugging
    //echo \'<pre>\'; print_r( ( 1 /*change to 0 to show src info*/ ) ? get_the_post_thumbnail( $ct_p_id[0], $size, $attr ) : wp_get_attachment_image_src( get_post_thumbnail_id( $ct_p_id[0] ) ) ); echo \'</pre>\'; 
    // now we can use this to return our thumbnail
    return get_the_post_thumbnail( $ct_p_id[0], $size, $attr );
}

echo wpse135208_cat_thumb_from_random_child( \'1\', \'thumbnail\', null );

$categories = get_categories($args);
foreach($categories as $category) { 
    echo wpse135208_cat_thumb_from_random_child($category->cat_ID); 
} 

<小时>Category thumbnail via plugin:

这样做的好处是具有实际的类别缩略图

我给了插件-Category Thumbnails - 我在上面的评论中提到了一个快速测试。一般来说，这是可行的，我觉得这是一个比你在回答中提出的更好的解决方案，因为它可以让你通过wordpress媒体库管理缩略图。

我发现了一个小错误，插件作者是notified 并立即回复，因此该错误将很快得到修复。如果你等不及了，你可以像这样自己修复它：

然而，功能

the_category_thumbnail();

function the_category_thumbnail($category_id = null, $sizes = array()) {
    print get_the_category_thumbnail($cat, $sizes);
}

function the_category_thumbnail($category_id = null, $sizes = array()) {
    print get_the_category_thumbnail($category_id, $sizes);
}

或者你只是使用get_the_category_thumbnail() 相反，你只需要echo 信息技术：

echo get_the_category_thumbnail(\'1\');

/**
 * wpse135208_cat_thumb_wrapper.
 *
 * Wrapper function to have the ability to use predefined image sizes via wp_get_attachment_image().
 *
 * @version 0.1
 *
 * @link https://wordpress.stackexchange.com/q/135208/22534
 *
 * @param integer $category_id (default: null)
 * @param string/array $size (default: \'thumbnail\')
 * @param boolean $icon (default: false)
 * @param string/array (default: null)
 *
 * @return string
 */
function wpse135208_cat_thumb_wrapper( $category_id = null, $size = \'thumbnail\', $icon = false, $attr = null ) {
    $category_thumbnail_obj = get_the_category_data( $category_id );
    return wp_get_attachment_image( $category_thumbnail_obj->id, $size, $icon, $attr );
}

echo wpse135208_cat_thumb_wrapper( \'1\', \'thumbnail\', false, null );

$categories = get_categories($args);
foreach($categories as $category) { 
    echo wpse135208_cat_thumb_wrapper($category->cat_ID); 
}