"I cannot connect to the Members database. "
That is an absolute error. That says your database info in the config-inc.php file for the user database is incorrect until you get past that there is nothing more I can say here.
Photopost works with ubbthreads 7 example here and several other users have upgraded
I can tell you once you get past that issue there are several steps you need to do as you need to switch integrations if this is an upgrade. Once you upgrade your photopost to the lastest code. You need to do this
1. make sure the config has the database info for ubbthreads 7 if it changed
2. update your settings table setting 41 and 83 with the proper integration type and mysql table prefix of ubb7 as these will have changed to threads7 and ubbt_ instead of threads and w3t_ that a normal threads would have.
3. You need to download the userfix script from the photopost import forum to realign photo userid's since I beleive the ubbthreads7 upgrade script changes user's userid's like vb's impex script does which is why we created that script in the first place.