To get post's featured image from any post in your WordPress Multisites network, use this function:
For example, get the featured image of post_id 2 of blog_id 3:
<?php $myfeaturedimage_src = wp_get_post_featured_image_src(2, 3, 'full'); ?>
<img src="<?php echo $myfeaturedimage_src; ?>">
 Original author: https://gist.github.com/bamadesigner/f5a338805d28f9c15df7
 Backup link: https://github.com/dangtrinh/WordPress_Multisites_Utilities_Plugin/blob/master/ssis_blog_plugin.php
Notes: This is my 500th post!!!