When you have a new post. you are lazy upload Feature Image. if you download and activate this plugin. you will be happy. Because this plugin will get your image in your post to show your post on your website
you should copy this code to your position code of thumbnails
<?php echo do_shortcode('[thumbnails]'); ?>
if you want to set about width and height
<?php echo do_shortcode('[thumbnails width="100" height="100"]'); ?>