Well integrating an already installed classifieds with photopost would be tough as the users would need to be copied to the photopost users table and users would probally already exist with the same userid so you need to figure how to combat that issue.
As far as integrating with Photopost on a normal install or classifieds or reviewpost you simply install and select external integration and pick photopost treating things on install as an external integration.
Basically to switch an existing site you would do this
1. make sure the users exist in the photopost users table
2. edit the config file to make sure the database info is correct to connect to photopost for user database
3. change settings 41 and 83 to the proper integration type and photopost table prefix for reviewpost and classifieds