If you’d like to hide the featured images on all posts in the Extra Theme, so that they show up only on the homepage, etc, you can do so with the following CSS:

.single .post-thumbnail.header { display: none; }

To add the CSS code to Extra, paste it into Extra's custom CSS box, which you can find from your WordPress admin menu at "Extra > Theme Options > General > Custom CSS". Paste it after any other code already in there. Alternatively, you can place it in the style.css file of your child theme (if using).

Please see this post if you only want to hide the featured images on particular posts.