Page’s don’t by default have a “Featured Image”. If you’ve found a plugin to do that, then to output the Featured Image use the following function:
the_post_thumbnail()
You can read more about it in the WordPress Codex here.
There are various other functions that will return the image markup (get_the_post_thumbnail ()), or just the Featured Image source (wp_get_attachment_image_src()). Again, look in the Codex for detailed instructions on how to use these functions.