try this:
in /includes/functions.php find:
global $site_sess, $config;
replace withglobal $site_sess, $config, $user_info;
then, find:if (!get_file_path($thumb_file_name, "thumb", $cat_id, 0, 0)) {
replace with:if (!get_file_path($thumb_file_name, "thumb", $cat_id, 0, 0) || $user_info['user_level'] == GUEST) {
this should disable all thumbnails from showing to GUESTs