Well the easiest to use would be the Shinra Online Point system. But for your request on making it somehow integrate with this PhotoPost thingy, I'm not too sure...you will have to know some PHP coding of yourself and manually do some hacking to your files. There is no hack on this forum that supports what you are looking for.
Lesane Store provides a point system and also a store where you can basically buy permissions. It doesn't come with this PhotoPost thing, once again, but with some hardcoding you could probably figure it out.
So to sum it up:
Shinra Online Point System = Fast to install, easy, simple.
Lesane Store = lengthy to install, much much more features.
Both do not provide Photopost add-ons of any kind, and you will have to hard code it yourself.
Good luck