You may only change your pic base during the week following a PPV.
That means that if you want to change your pic base you should reserve it here so that nobody else takes the pic base you want before that time.
You can only reserve one pic base at a time. This pic base is yours unless you do not post new sign-up information in the week after a PPV. If you do not it will become available again.
If you do not sign up with your reserved pic base 2 days after the availability, the pic base will be open to the roster and you will have to choose someone else.