The author did not provide what you want.
Therefore, load only the thumbnails. Then there will be one image.
I have already asked the author to do, but so far there is no answer:
https://wordpress.org/support/topic/post-featured-image-width100-height100?replies=7
Theme Options/ Design / Posts page listing display type (Large featured image /or/ Small featured image)
But to display one image. It is necessary that the author did:
/wp-content/themes/accelerate/content-single.php
<?php
/**
* The template used for displaying page content in page.php
*
* @package ThemeGrill
* @subpackage Accelerate
* @since Accelerate 1.0
*/
?>
<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
<?php do_action( 'accelerate_before_post_content' ); ?>
<meta property="og:image" content='<?php the_post_thumbnail('full'); ?>'/>
<meta itemprop="image" content='<?php the_post_thumbnail('full'); ?>'/>
<meta name="twitter:image:src" content='<?php the_post_thumbnail('full'); ?>'/>
<link rel="image_src" href='<?php the_post_thumbnail('full'); ?>'/>
<div id="post-header-image-v1" style="">
<?php the_post_thumbnail('full'); ?>
</div>
<div id="post-header-text-v1" style="">
<header class="entry-header">
<h1 class="entry-title">
<?php the_title(); ?>
</h1>
</header>
<?php accelerate_entry_meta(); ?>
<div class="entry-content clearfix">
<?php
the_content();
wp_link_pages( array(
'before' => '<div style="clear: both;"></div><div class="pagination clearfix">'.__( 'Pages:', 'accelerate' ),
'after' => '</div>',
'link_before' => '<span>',
'link_after' => '</span>'
) );
?>
</div>
<?php do_action( 'accelerate_after_post_content' ); ?>
</article>
