HI,
you can do this by using paypal subscription buttons if you charge monthly if not you can charge a one-time fee...
Basically unless you really tear apart and rebuild everything you will have to do it like this...
Let them sign up....Paypal gives you the option of letting them have a trial period...set it to 1 or 2 days...(enough time for you to approve them once they pay)...let the user know that they have 1 or 2 days...BUT make sure they know that to activate their account they have to click on a button on your main page....This button will be coded to let them make a payment to you thru paypal...
By doing it this way once they pay you can bounce that email address that you get from paypal that they paid with off of their email in their user profile...once the 2 emails match activate their account in 4images...send them a thank you email...and you're done..
Of course all this CAN be automated but that is a paypal issue...go to paypal.com and at the bottom go to the link "Developers" and read up on it ...but this gives you a quick fix till then..
Take Care,
Joey