Картинки выводятся в категории, там это реализовано так:

..../category.tpl

<div class="well red-links">
        <?php if ($thumb) { ?>
        <div class="pull-left"><img src="<?php echo $thumb; ?>" alt="<?php echo $heading_title; ?>" title="<?php echo $heading_title; ?>" class="img-thumbnail" style="margin: 0 10px 5px 0" /></div>
        <?php } ?>
        <?php if ($description) { ?>
        <?php echo $description; ?>
        <?php } ?>
				<div class="clearfix"></div>
      </div>

..../category.php

....
$this->load->model('tool/image');
....
			if ($category_info['image']) {
				$data['thumb'] = $this->model_tool_image->resize($category_info['image'], $this->config->get('config_image_category_width'), $this->config->get('config_image_category_height'));
			} else {
				$data['thumb'] = '';
			}
...
			foreach ($results as $result) {
				if ($result['image']) {
					$image = $this->model_tool_image->resize($result['image'], $this->config->get('config_image_product_width'), $this->config->get('config_image_product_height'));
				} else {
					$image = $this->model_tool_image->resize('placeholder.png', $this->config->get('config_image_product_width'), $this->config->get('config_image_product_height'));
				}
...
$data['products'][] = array(
					'product_id'  => $result['product_id'],
					'thumb'       => $image,
					'name'        => $result['name'],
					'description' => utf8_substr(strip_tags(html_entity_decode($result['description'], ENT_QUOTES, 'UTF-8')), 0, $this->config->get('config_product_description_length')) . '..',
					'price'       => $price,
					'special'     => $special,
					'tax'         => $tax,
					'minimum'     => $result['minimum'] > 0 ? $result['minimum'] : 1,
					'rating'      => $result['rating'],
					'href'        => $this->url->link('product/product', 'path=' . $this->request->get['path'] . '&product_id=' . $result['product_id'] . $url)
				);

Пробовал так вот еще:
..../catalog/controller/common/header.php
После:
$this->load->model('catalog/product');
Добавил:
$this->load->model('tool/image');
После:
foreach ($categories as $category) {
Добавил:

if ($category['image']) {
$image = $this->model_tool_image->resize($category['image'], 100, 50);
} else {
$image = false;					
}

После:

'href'     => $this->url->link('product/category', 'path=' . $category['category_id']),

Добавил:
'thumb' => $image
В файле .../common/header.tpl
Выводил так:

<img src="<?php echo $category['thumb']; ?>" border="0">