Finn,
Your server is running in safe mode which cripples file uploads. Our program requirements state safe mode must be OFF to run the program. You might need to take this up with your host. Let me link a couple things here. One is going to be the program requirements and the next is the installation instructions which state the directory and file permissions that need to be set
http://www.photopost.com/requirephp.html http://www.photopost.com/installphp.html