[Theme Horse Support Team]

Hi RWM2718,
Our default is blog image large. So blog image medium will not work for category. It needs some code customziation. Go to theme folder -> library -> functions -> functions.php -> content-extensio.php on line no 384 you will find below code

$image .= get_the_post_thumbnail( $post->ID, 'featured', array( 'title' => esc_attr( $title_attribute ), 'alt' => esc_attr( $title_attribute ) ) ).'</a>';
replace with
$image .= get_the_post_thumbnail( $post->ID, 'featured-medium', array( 'title' => esc_attr( $title_attribute ), 'alt' => esc_attr( $title_attribute ) ) ).'</a>';

We recommended not to change the code because if you change the code then while updating to our new version all your customisation code will be lost. So better make child theme and first unhook the functions and the edit the code.

http://codex.wordpress.org/Child_Themes

Thank you