Display Page featured Image as well as Posts featured Image

WordPress loop functions depend on the global variables, so you will have to get the data from page before you run the loop.

Generally, you should be able to do something like this-

$page_id = get_the_ID(); //this is important, we will use this later

while(have_posts()): the_post();
//do something with the post here

echo get_the_post_thumbnail( $page_id ); //this is the page featured image

//still displaying any post related data
endwhile;