Add custom field to Item and Album Tables

Hello

I´ve tested Gallery2 in combining with Drupal.
I´m realy suprised how good this both scripts are working together.
I´ve already coded a Drupal block which let my users manage their Albums and Items my way. But know I want to do the following:

I want to add a custom field into the Database Table of Items.
This is because I want that users can expect "User Points" for their Pictures.
That means, everyone how Uploads a Picture or an Album full of Pictures can set an int value. So someone clicks on the picture to see in fullsize he will pay the amount of "User Points" set by the int value automaticly.

For this I want to use the Drupal module "User Points".

Has anybody an idea to realise this task as easy as possible ?

It would be nice getting help here...

Kind regards

Uhu