@sallen_acs just in case you still didn't get it right, I found the correct naming for images on Prestashop 1.7.2 (possibly it works on 1.7.3 and higher):
<img class="replace-2x" src="{$urls.base_url}img/c/{$subcategory.id_category}-small_default.jpg" alt="{$subcategory.name|escape:'html':'UTF-8'}" />