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 Settings > General > Custom CSS". Paste it after any other code already in there.

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