If you recieved an email for a photoCD, it's a prepurchase special offer.
It's a random email for those staying on property. It's up to you to pre-purchase. But if you've used it before and like it. It's not a bad deal to save 20% off of it.
If you like meeting characters, you'll run into photopass alot. If you're more about the icons, you might have a harder time because characters have a higher priority than icons, so some one might get pulled to do something else. But sometimes if the character is going away for 20-30mins, the photographer will wander off someplace near-by to take pictures.