Trying to put together a Credits System that keeps track of Client Purchases?

hi,
I want to implement a fairly simple Client / Credits Scoring system that keeps
track of Users purchases and how many credits the client has remaining.

So far I have created the following.

Created a table called credits as below

CreditId (int) Pk
Amount (int)
ClientId (GUID)
PurchaseDate (Datetime)

On joining Users fill in the registration form on my site and select 25, 50 , or 100 credits from a checkbox list.
The user clicks submit and their name , address is inserted into Users Table and
2,5, 50, or 100 int value goes to the CreditsTable and their ClientId.

On Campaign.aspx.
I have a gridivew that displays images from a database table Images.
next to the images column in the gridvidew  is a checkbox.
Lets say there are 50 images in the gridview and they select all the checkb ...