The changes set forth below will cause OpenCart to assign a random product image from the category as the category image if one has not already been set. If the category contains no products, no_image.jpg will still display.

Make the following changes around line 112 of catalog/controller/product/category.php.

foreach ($results as $result) {
	if ($result['image']) {
		$image = $result['image'];
	} else {
/* START AUTOMATIC CATEGORY IMAGES */
      		$catimg = $this->model_catalog_product->getRandomProductImageByCategoryId($result['category_id']);
      		if($catimg){
      			$image = $catimg['image'];
      		}else{
      			$image = 'no_image.jpg';
      		}
/* END AUTOMATIC CATEGORY IMAGES */
	}
	
	$this->data['categories'][] = array(
		'name'  => $result['name'],
		'href'  => $this->model_tool_seo_url->rewrite(HTTP_SERVER . 'index.php?route=product/category&path=' . $this->request->get['path'] . '_' . $result['category_id'] . $url),
		'thumb' => $this->model_tool_image->resize($image, $this->config->get('config_image_category_width'), $this->config->get('config_image_category_height'))
	);
}

Add this function to catalog/model/catalog/product.php.

public function getRandomProductImageByCategoryId($category_id) {
	$sql = "SELECT p.image FROM " . DB_PREFIX . "product p LEFT JOIN " . DB_PREFIX . "product_to_category p2c ON (p.product_id = p2c.product_id) WHERE p.status = '1' AND p.image != '' AND p.date_available <= NOW() AND p2c.category_id = '" . (int)$category_id . "' ORDER BY RAND() LIMIT 1";
			
	$query = $this->db->query($sql);
									  
	return $query->row;
}

That’s it, enjoy your automatically generated and random category images!