Featured images are used differently depending on the theme. In frontier’s case it is only used for excerpt thumbnails and on sliders.
If you want to add one, you can add the code below on a PHP Code Widget then place it on either the Post – Header or the Post – Before Content widget area.
<?php the_post_thumbnail( 'full' ); ?>
http://codex.wordpress.org/Function_Reference/the_post_thumbnail