Hi Jacob,
thank you very much! I found the problem in content.php, where I had to replace ‘post-thumbnails’ with ‘martin-home’:
$image .= get_the_post_thumbnail( $post->ID, ‘martin-home’, array( ‘title’ => esc_attr( $title_attribute ), ‘alt’ => esc_attr( $title_attribute ) ) ).’‘;
and to add this to functions.php:
add_theme_support( ‘post-thumbnails’ );
add_image_size( ‘martin-home’, ‘462’, ‘0’, true );