Again, it depends what database means. You seem to think that a database is one particular thing, and it isn't. It depends fundamentally on the data structure involved. The implicit assumption in that SO question is that database means SQL database, but it doesn't. (Though that was kinda reasonable to assume in 2012.) The Firebase database is NoSQL for example, and the SO question you posted includes a link to a description of the Haystacks data structure used by Facebook in their photo storage.
But if you want comments on storing all photos on the app, I think it's fair to point out that unless your target user puts a high priority on secrecy, then the user experience you'd be providing is less than what people now expect from apps. People expect that they can "clear the cache" and still recover their personal data. So you need what would be understood by a user as "cloud storage," or people might be unpleasantly surprised.