Este é um problema complicado e difícil de explicar.
Estou a usar o tema Manganescent, estou também a integrar o WP Nivo Slider no tema, mas as thumbanails no nivo slider ficam sempre pequenas.
O tema tinha no ficheiro functions.php uma funções para as thumbnails serem geradas pelo plugin Regenerate Thumbnails.
functions.php
<?php
define('PADD_THEME_NAME','Manganescent');
define('PADD_THEME_VERS','1.0');
define('PADD_THEME_SLUG','manganescent');
define('PADD_NAME_SPACE','padd');
define('PADD_GALL_THUMB_W',400);
define('PADD_GALL_THUMB_H',260);
define('PADD_LIST_THUMB_W',140);
define('PADD_LIST_THUMB_H',140);
define('PADD_YTUBE_W',255);
define('PADD_YTUBE_H',180);
define('PADD_THEME_FWVER','2.6.1');
define('PADD_THEME_PATH',get_theme_root() . DIRECTORY_SEPARATOR . PADD_THEME_SLUG);
define('PADD_FUNCT_PATH',PADD_THEME_PATH . DIRECTORY_SEPARATOR . 'includes' . DIRECTORY_SEPARATOR);
remove_action('wp_head','wp_generator');
add_theme_support('post-thumbnails');
add_theme_support('automatic-feed-links');
register_nav_menus(array('main' => 'Main Menu'));
set_post_thumbnail_size(PADD_LIST_THUMB_W,PADD_LIST_THUMB_H,true);
add_image_size(PADD_THEME_SLUG . '-thumbnail',PADD_LIST_THUMB_W,PADD_LIST_THUMB_H,true);
add_image_size(PADD_THEME_SLUG . '-gallery',PADD_GALL_THUMB_W,PADD_GALL_THUMB_H,true);
function padd_widgets_init() {
register_sidebar(array(
'name' => 'Sidebar',
'before_widget' => '<div id="%1$s" class="box %2$s">',
'after_widget' => '</div></div>',
'before_title' => '<div class="title"><h3>',
'after_title' => '</h3></div><div class="interior">',
));
register_sidebar(array(
'name' => 'Footer 1',
'before_widget' => '<div id="%1$s" class="box %2$s">',
'after_widget' => '</div></div>',
'before_title' => '<div class="title"><h3>',
'after_title' => '</h3></div><div class="interior">',
));
register_sidebar(array(
'name' => 'Footer 2',
'before_widget' => '<div id="%1$s" class="box %2$s">',
'after_widget' => '</div></div>',
'before_title' => '<h3>',
'before_title' => '<div class="title"><h3>',
'after_title' => '</h3></div><div class="interior">',
));
register_sidebar(array(
'name' => 'Footer 3',
'before_widget' => '<div id="%1$s" class="box %2$s">',
'after_widget' => '</div></div>',
'before_title' => '<h3>',
'before_title' => '<div class="title"><h3>',
'after_title' => '</h3></div><div class="interior">',
));
register_sidebar(array(
'name' => 'Footer 4',
'before_widget' => '<div id="%1$s" class="box %2$s">',
'after_widget' => '</div></div>',
'before_title' => '<h3>',
'before_title' => '<div class="title"><h3>',
'after_title' => '</h3></div><div class="interior">',
));
}
add_action('widgets_init','padd_widgets_init');
require PADD_FUNCT_PATH . 'library.php';
require PADD_FUNCT_PATH . 'hooks.php';
require PADD_FUNCT_PATH . 'classes' . DIRECTORY_SEPARATOR . 'socialnetwork.php';
require PADD_FUNCT_PATH . 'classes' . DIRECTORY_SEPARATOR . 'socialbookmark.php';
require PADD_FUNCT_PATH . 'classes' . DIRECTORY_SEPARATOR . 'advertisement.php';
require PADD_FUNCT_PATH . 'classes' . DIRECTORY_SEPARATOR . 'widgets.php';
require PADD_FUNCT_PATH . 'classes' . DIRECTORY_SEPARATOR . 'twitter.php';
require PADD_FUNCT_PATH . 'classes' . DIRECTORY_SEPARATOR . 'pagination.php';
require PADD_FUNCT_PATH . 'classes' . DIRECTORY_SEPARATOR . 'walker.php';
require PADD_FUNCT_PATH . 'classes' . DIRECTORY_SEPARATOR . 'input' . DIRECTORY_SEPARATOR . 'input-option.php';
require PADD_FUNCT_PATH . 'classes' . DIRECTORY_SEPARATOR . 'input' . DIRECTORY_SEPARATOR . 'input-socialnetwork.php';
require PADD_FUNCT_PATH . 'classes' . DIRECTORY_SEPARATOR . 'input' . DIRECTORY_SEPARATOR . 'input-advertisement.php';
require PADD_FUNCT_PATH . 'defaults.php';
require PADD_FUNCT_PATH . 'administration' . DIRECTORY_SEPARATOR . 'options-functions.php';
require PADD_FUNCT_PATH . 'administration' . DIRECTORY_SEPARATOR . 'posting-functions.php';
require PADD_FUNCT_PATH . 'launch.php';
Já removi algumas coisas, mas deixo aqui o original.