We just did this a couple weeks ago. It is called Attractions+ and it's just under $50 (well, just over once the tax is added on). You can purchase it at any ride photo claim counter, so after your first ride, just buy it and you'll get a card that you present after every ride to add them to your account.
One thing that was a little inconvenient is that you have to have the physical card to claim the photo, so if your group splits up, make sure the ones riding the photo ride have the card. If you forget, you can go back with the photo number and claim your photo later.
No limits on how many photos you can claim and add to your account.
The only issue we had was with Hollywood Studios photos, and it was fixable. On day 1, we added the Attractions+ photo card number to my Photopass online account and from that point EXCEPT from HS. For those to appear, I had to re-add the Attractions+ card number to my account.

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Over all, I'm pleased with Attractions+, however, I assumed that when I downloaded the photos, I would have access to the same neat photo borders that you can get on the photos at the ride. Nope. There is a border available for each ride photo, but it's pretty plain. Ride name, year, and a color around the edge. Kindof bummed about that.