{* Assign a category image variable with physical path to file on server - NOT browser path! *}
{assign var="category_thumbnail" value="/home/cpanelaccountusername/public_html/img/c/{$GetCategoryIdHere|escape:'html':'UTF-8'}_thumb.jpg"}

{* Check if category thumbnail file path exists for this category and show the correct image *}
{if file_exists($category_thumbnail)}
	<img class="img-responsive" src="{$img_ps_dir}c/{$GetCategoryIdHere|escape:'html':'UTF-8'}_thumb.jpg" alt="{$GetCategoryTitleHere|escape:'htmlall':'UTF-8'}">
{/if}