Installed Okay but have this error
 

Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource in /photopost.php on line 25

How can I fix this?

Sorry here your thread got lost here. That error tells me that you have entered the incorrect table prefix for photopost when you installed the script to integrate with your photopost on step 4 where it asks for table prefix etc. That or the user database(photopost) has the wrong info for photopost database.