Theme Horse

Hi Ekweb
Create child theme and unhook the function of archive from library->structure->content-0xtension.php and write the same function and you can change the image. In archive page there is blog image large. So you need to fix it to blog image medium.

function interface_theloop_for_archive() {
	global $post;

	if( have_posts() ) {
		while( have_posts() ) {
			the_post();

			do_action( 'interface_before_post' );
?>
	<section id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
	 				<?php do_action( 'interface_before_post_header' ); ?>
	 	<article>
								<?php
				if( has_post_thumbnail() ) {
					$image = '';        			
		     		$title_attribute = apply_filters( 'the_title', get_the_title( $post->ID ) );
		     		$image .= '<figure class="post-featured-image">';
		  			$image .= '<a href="' . get_permalink() . '" title="'.the_title( '', '', false ).'">';
		  			$image .= get_the_post_thumbnail( $post->ID, 'featured', array( 'title' => esc_attr( $title_attribute ), 'alt' => esc_attr( $title_attribute ) ) ).'</a>';
					$image .='<span class="arrow"></span>';
		  			$image .= '</figure>';

		  			echo $image;
		  		}
	  			?>

In this line
$image .= get_the_post_thumbnail( $post->ID, 'featured', array( 'title' => esc_attr( $title_attribute ), 'alt' => esc_attr( $title_attribute ) ) ).'</a>';
instead of featured you keep featured-medium
Regards
Sunil