You were running v5.13; we are up to v5.3, so I would suggest upgrading. I wouldn't be able to tell what kind of exploit may have been used or even if it came from PhotoPost without shell access to review your server logs.
The hacker replaced your config-inc.php file with some code to print that page out; I replaced your config-inc.php and you just need to upload your old copy or edit this one with your db info.
Everything else appears to be normal; but without shell access to review your logs, its difficult for me to guess what may have caused this.
Please do not PM me for support or sales questions. Thank you for your understanding.