Lo primero que debes realizar es definir los tamaños de las imágenes en el functions.php de tu theme

if (function_exists('add_theme_support')) {
add_theme_support('post-thumbnails');
add_image_size('imagenportada', 620, 310, true);
}

 

Para mostrar las miniaturas en el home:

$Noticias = get_posts(array(
'posts_per_page' => 3, //cantidad
'offset' => 0,
'category' => '24',//categoria
'orderby' => 'post_date',
'order' => 'DESC',
'include' => '',
'exclude' => '',
'meta_key' => '',
'meta_value' => '',
'post_type' => 'post',
'post_mime_type' => '',
'post_parent' => '',
'post_status' => 'publish',
'suppress_filters' => true)
);
$i = 0;
foreach ($Noticias as $post) : setup_postdata($post);
$image = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID), 'imagenportada');
?>

<?php the_title(); ?>