I can not tell you. If you can not link a phpinfo file here that we can look at I can not be sure. Our requirements page lists the requirements to run Photopost on your server. It is up the user to make sure there server is compatible. If you linked a phpinfo file for me to look at I may be able to tell you your issue however you can not do that.
Hense my post that you need to contact your host and find out your server configuration and whether it meets the requirements to run Photopost found on this page
http://www.photopost.com/requirephp.html