Check, thanks for your advice so far. Now I have purchased PhotoPost and am busy with the installation. I have got to the last stage and go the message:
The final installation step will help you begin configuring PhotoPost for your server. Now you must choose whether you intend to use PhotoPost's Internal registration system, or a third party's, to register yourself and your users to grant access to upload photos and post comments. The two options are:
a) PhotoPost Internal User Registration
b) External User Registration
If I want existing blog users to be able to login with their existing usernames and passwords, should I proceed with a) or b)?