Thanks. It seemed to me in the beginning that HIDDEN fields were a very clumsy solution. Now I understand why they indeed are.

We use PHP as our server side scripting language. I was thinking of using the $_SESSION variable but attributes and pointer information for 30 image uploads per user * no of concurrent users seems like a huge drag on server memory.

I'll probably just write the pointer information to the database for now after each upload. It might make the web app slower by a few milliseconds but it reduces unnecessary complications