https://github.com/crstauf/WordPress-FeaturedImage-SpecialImageSize
试试这个；很久以前我自己写的。我有一个更新版本，很快就会发布。如果你有任何问题，请告诉我。
根据要求，功能的相关代码。主题的php：
// `post_type` => array(width,height,crop)
$featimg_sizes = array(
\'post\' => array(50,20,false)
);
add_action(\'wp_ajax_set-post-thumbnail\',\'generate_featimg_size\',1);
function generate_featimg_size() {
global $featimg_sizes;
$thumbnail_id = intval( $_POST[\'thumbnail_id\'] );
$type = get_post_type(intval( $_POST[\'post_id\'] ));
if (!isset($thumbnail_id)) return;
if (\'-1\' == $thumbnail_id) return;
if (!array_key_exists($type,$featimg_sizes)) return;
$size = $featimg_sizes[$type];
if (!isset($size[0]) || !isset($size[1]) || empty($size[0]) || empty($size[1])) return;
if (!isset($size[2])) $size[2] = false; // set $crop if left blank to false
$meta = wp_get_attachment_metadata($thumbnail_id);
$path = apply_filters(\'image_make_intermediate_size\',get_attached_file($thumbnail_id));
if ($newsize = image_make_intermediate_size($path,$size[0],$size[1],$size[2])) {
$backupsizes = get_post_meta($thumbnail_id,\'_wp_attachment_backup_sizes\',true);
$meta[\'sizes\'][\'ptfi_\'.$type] = $backupsizes[\'ptfi_\'.$type] = $newsize; // ptfi = post type featured image
update_post_meta($thumbnail_id,\'_wp_attachment_metadata\',$meta);
update_post_meta($thumbnail_id,\'_wp_attachment_backup_sizes\',$backupsizes);
}
}
`
