Solucionado con este código:
Código PHP:
SELECT
p1.*,
wm2.meta_value
FROM
wp_posts p1
LEFT JOIN
wp_postmeta wm1
ON (
wm1.post_id = p1.id
AND wm1.meta_value IS NOT NULL
AND wm1.meta_key = "_thumbnail_id"
)
LEFT JOIN
wp_postmeta wm2
ON (
wm1.meta_value = wm2.post_id
AND wm2.meta_key = "_wp_attached_file"
AND wm2.meta_value IS NOT NULL
)
WHERE
p1.post_status="publish"
AND p1.post_type="post"
Gracias de nuevo.