I am also having a problem similar in nature with bridging CPG to Joomla. Initially, my Joomla users will log in to CPG. I found info about modifying your mod_login.php file in Joomla so that Remember Me is not a checkbox but is changed to a hidden input field. This works and when a Joomla user logs in, they will automatically be logged in to CPG.
The problem I have, is they cannot log out of CPG. When they log out of Joomla, that works fine, but they remain logged in to CPG. If they then log out of CPG using its log out feature, they will not be able to log in again when they log in to Joomla. They only solution is to delete the site's cookies. They can then log in to Joomla once more and they will be logged in to CPG as well again.
For single users that want to remain logged in, this is not a problem. But if it's a work computer, or if there is more than one person using the computer, and logging out is necessary, it's a huge problem. Has anyone successfully bridged just Joomla and CPG?
http://forum.coppermine-gallery.net/index.php?topic=24032.0