上周也有同样的问题，下面是我所做的：

if (has_post_thumbnail()) 
    //if the post already has a post thumbnail then just use that
    the_post_thumbnail($size = \'post-thumbnail\', $attr = \'\');
else{
    //if not then convert the custom field to the post thumbnail and display that
    $at_url = get_post_meta($post->ID, \'image\', true);
    $at_id = get_image_id_by_url($at_url);
    delete_post_meta($post->ID, \'image\');
    if ($at_id){
        set_post_thumbnail($post, $at_id);
        the_post_thumbnail($size = \'post-thumbnail\', $attr = \'\');
    }else{
        //else just display a default image or not :)
    }
}