I would think its a cookie issue. If you know how to use phpmyadmin view the photopost settings table and check out settings 171-173 which are the cookie settings or rerun install.
Basically in your config-inc.php file make sure you have entered the correct vbulletin license number.
Then in your vbulletin admin cookie settings and in the Photopost cookie settings make them this
cookie path /
cookie prefix bb
cookie domain .domain.com