◆page.phpなど
<?php
$thumbnail_id = get_post_thumbnail_id();
$src_info = wp_get_attachment_image_src($thumbnail_id, 'full');// 『thumbnail,medium,large,full』を指定可
$src = $src_info[0];
if(!empty($src)){
$img_src = $src;
}else{
$img_src = "";
}
?>
<img src="<?php echo $img_src; ?>" height="718" width="1920" alt="" class="" />