Simply convert the integration type in your photopost settings table with phpmyadmin or similar utility and find setting #41 and set it to postnuke. Also do not forget if you use any forum prefix on your postnuke tables to set #83 to this in your settings table.
You will need to match up any cookie settings between both products as in
cookie set to worldwide /
cookie domain: .domain.com
cookie prefix: if any
must all match between the two.