Not sure if this has been answered, but theres a bug when attempting to logout via 4images in phpbb 2.0.+..
This can be resolved by opening includes/sessions.php in your 4images folder.
Find "$url_logout"
In line find:
?logout=true
After inline add:
&sid=".$_COOKIE["phpbb2mysql_sid"]."
It should look like this when your done:
$url_logout = $url_app."login.php?logout=true&sid=".$_COOKIE["phpbb2mysql_sid"]."";
What where doing is pulling the cookie SID for phpbb2 and placeing it in the logout link in 4image.
There was also another method which would be to modify the phpbb2 login.php page which removes the sid check,
this is not recomended, and can create a security hole..
Hope this helps,
Gordon