Twenty Twelve WordPress Theme sets a big old ‘Featured Image’ on top of the post content if the featured image is set in a post.
To remove the Featured Image from this area but keep it intact for excerpts where it shows elsewhere you need to hack it in CSS or edit the content.php file, working in a child theme make a copy of content.php from the parent 2012 to your child theme.
Go to line 20 and delete or comment out
<?php the_post_thumbnail(); ?>
If you have a blog page set to a page other than the home page then you also need to make the same edit to the content-page.php file to get rid of the featured image on line 14