Couple points that might help.
1. Make sure to go to the admin of both phpbb and photopost and make sure the cookies are set exactly the same. Make sure the cookie domain is .domain.com on both so it works on every subdomain.
2. Edit your phpbb templates index_body and login_body templates I beleive and make this line here and add the code in bold so it is checked automatically
Content visible to verified customers only.