I think you got the right idea. Add new columns to the database (pictures table) to help describe the extra meta data you want to use. You don't have to modify delete picture, but you will need to edit add_picture() and update_picture(), expanding the SQL queries to accomodate your new variables.
Next, you'll want to edit gallery.php and choose how you want to output those variables into your layout.
You also may want to edit plog-manage.php to allow you to edit entries with the new variables.
Obviously, this is a very high level description of what you need to do. To get this right, it is potentially a lot of work. Good luck, let us know how it goes.