I'm sorry, put an extra comma at the end:
$sql = "SELECT c.image_id, c.comment_id, c.user_id as comment_user_id, c.user_name as guest_user_name, c.comment_headline, c.comment_text, c.comment_date, i.cat_id, i.user_id, i.image_name, i.image_media_file, i.image_thumb_file".get_user_table_field(", u.", "user_name")." as user_name".get_user_table_field(", s.", "user_name")." as comment_user_name, $additional_sql
Correct line is:
$sql = "SELECT c.image_id, c.comment_id, c.user_id as comment_user_id, c.user_name as guest_user_name, c.comment_headline, c.comment_text, c.comment_date, i.cat_id, i.user_id, i.image_name, i.image_media_file, i.image_thumb_file".get_user_table_field(", u.", "user_name")." as user_name".get_user_table_field(", s.", "user_name")." as comment_user_name $additional_sql
I've updated the original post with the correct line.