$categories = $this->model_catalog_category->getCategories(0);
foreach ($categories as $category) {
$children_data = array();
.......$category_info = $this->model_catalog_category->getCategory($category_id);
.....
if ($category_info['image']) {
$data['thumb'] = $this->model_tool_image->resize($category_info['image'], $this->config->get('theme_' . $this->config->get('config_theme') . '_image_category_width'), $this->config->get('theme_' . $this->config->get('config_theme') . '_image_category_height'));
} else {
$data['thumb'] = '';
}