The stock version of product_listing.php has the output building code around line 99:
PHP Code:
case 'PRODUCT_LIST_MANUFACTURER':
$lc_align = '';
$lc_text = '<a href="' . zen_href_link(FILENAME_DEFAULT, 'manufacturers_id=' . $listing->fields['manufacturers_id']) . '">' . $listing->fields['manufacturers_name'] . '</a>';
break;
You would have to get the image filename/location into something like $listing->fields['manufacturers_image'], by modifying the SQL select statement in a previous file, probably /includes/modules/pages/index/main_template_vars.php (no override available).