So, if you’re excited about the new WordPress theme, but can’t get the featured images to display right on the featured posts, don’t worry. I had the same problem with one of my sites. It kept using the thumbnail instead of the full-size image.

To fix this problem you need to edit content-featured-post.php. Find the code that looks like this:

if ( has_post_thumbnail() ) :
if ( ‘grid’ == get_theme_mod( ‘featured_content_layout’ ) ) {
the_post_thumbnail();
} else {
the_post_thumbnail( ‘twentyfourteen-full-width’ );
}
endif;

Then add the bolded code.

if ( has_post_thumbnail() ) :
if ( ‘grid’ == get_theme_mod( ‘featured_content_layout’ ) ) {
the_post_thumbnail(‘twentyfourteen-full-width’);
} else {
the_post_thumbnail( ‘twentyfourteen-full-width’ );
}
endif;

Upload to your child theme and you’re done!