You can accomplish this one of two ways, depending on how comfortable you are editing theme files or CSS.
1) Quick-and-easy: Add CSS to hide the feature image area on single posts. This code should work:
.single-post .site-header-image {
display: none;
}
2) More labor intensive, but still not hard: Create a child theme (best practice and in case the theme updates), and copy over the header.php file and modify so that the following existing code:
<?php if (is_home()) : ?>
<section id="content" class="blog-home-content">
<?php else : ?>
<section class="site-header-image">
<?php // Check if this is a post or page, if it has a thumbnail, and if it's a big one
if ( is_singular() && has_post_thumbnail( get_the_ID() ) ) :
// Houston, we have a new header image!
//echo get_the_post_thumbnail( $post->ID, 'magnus-large' );
$image_id = get_post_thumbnail_id();
$url = wp_get_attachment_image_src( $image_id, 'magnus-large' ); ?>
<div class="section-image" style="background-image: url(<?php echo esc_attr( $url[0] ); ?>);">
</div><!-- .section-image -->
<?php elseif ( get_header_image() ) : ?>
<img src="<?php header_image(); ?>" width="<?php echo HEADER_IMAGE_WIDTH; ?>" height="<?php echo HEADER_IMAGE_HEIGHT; ?>" alt="" />
<?php endif; // end check for featured image or standard header ?>
</section><!-- .site-header-image -->
<section id="content" class="site-content">
<?php endif; // end check for blog homepage ?>
Gets changed to this:
<?php if (is_home()) : ?>
<section id="content" class="blog-home-content">
<?php else : ?>
<section id="content" class="site-content">
<?php endif; // end check for blog homepage ?>