If you never save a Prompto photo, what happens at the end?

Solution 1:

You can't get through the game with no pictures saved, as two are saved automatically.

From multiple users in this thread, it appears that you will always have the group picture from the beginning and the picture taken before the group departs for Altissia.

there are always 2 choices (one with the 4 guys, and another from just before you depart for artissia on the boat with cid and the cape caem guys)

There are some Prompto photos that are standard for everyone at key story events. It's likely that in the event you have none saved, it'll load these for you to look through and choose. I don't think they'd spend time with animating that section or the end credits if it was going to be completely blank.

I saved zero pictures and @imitebmike Is correct; there are two default photos that are automatically saved (the two he mentions)