Actually error 500 is a generic server error which mean usually you need to look at the server error logs to tell more of what the problem is
I would suggest you look at this and go from there.
You sure PHP is running on your server. Example like powweb for one host you need to enable cgi before php is active and if not you will get an error 500
So definately more info is needed here
Url to Photopost and if possible url to phpinfo file to see php is running?