WPDB Multiple site’s posts and get featured images

This is one way how to get the postmeta of the posts, by joining the postmeta table and fetching the _wp_attached_file meta value.

Try this:

$mydb->get_results( select a.*, thumb.meta_value 
from wp_posts a 
left join (select b.post_id, c.meta_value 
from wp_postmeta b, wp_postmeta c 
where b.meta_key = '_thumbnail_id' 
and b.meta_value = c.post_id 
and c.meta_key = '_wp_attached_file') as thumb 
on thumb.post_id = a.ID 
where a.post_type="post" 
and a.post_status="publish", OBJECT );